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Psychometric properties of the WHO Violence Against Women instrument in a female population-based sample in Sweden: a cross-sectional survey

Objective To explore psychometric properties of the Violence Against Women instrument in a randomly selected national sample of women (N=573) aged 18–65 years and residing in Sweden. Design Cross-sectional survey study. Setting Sweden. Participants A postal survey was sent to 1006 women between January and March 2009, during which 624 women (62%) returned the questionnaire. 51 women who did not answer any of the violence items were excluded from the analyses, resulting in a final sample of 573 women. Primary and secondary outcome measures Self-reported exposure to psychological, physical and sexual intimate partner violence. Results Cronbach's α coefficients were 0.79 (psychological scale), 0.80 (physical scale), 0.72 (sexual scale) and 0.88 (total scale). A predetermined three-component solution largely replicated the explored three component conceptual model of the Violence Against Women instrument. The instrument was able to discriminate between groups known from previous studies to differ in exposure to physical and/or sexual violence, that is, respondents with poor versus good self-rated health and witnessed versus not witnessed physical violence at home when growing up. Past-year prevalence of physical (8.1%; 95% CI 5.9 to 10.3) and sexual (3%; 1.6 to 4.4) violence was similar to that reported in other Nordic studies; however, earlier-in-life prevalence was lower in the current study (14.3%; 95% CI 11.4 to 17.2 and 9.2%; 95% CI 6.8 to 11.6, respectively). Reported exposure rates were higher than those obtained from a concurrently administered instrument (NorVold Abuse Questionnaire). Conclusions The Violence Against Women instrument demonstrated good construct validity and internal reliability in an adult female population in Sweden. However, further studies examining these and other psychometric properties need to be conducted in other countries.


INTRODUCTION
While prevalence studies investigating violence against women perpetrated by intimate male partners have become more frequent, (1)(2)(3)(4) sizeable differences in reported exposure occur both between and within study sites. These differences may in part be explained by differences in questionnaire administration methods (e.g. personal interviews vs. selfadministration), questionnaire content, target populations, or definitions and severity of the violence assessed; however, such differences may also reflect true variation and cultural differences in violence perpetration. (1,4) Standardized methodologies for assessing intimate partner violence (IPV) may help to enhance the reliability of results obtained from such studies and aid in comparing prevalence rates from diverse settings.
With this in mind, the World Health Organization (WHO) constructed a questionnaire for the WHO Multi-country Study on Women's Health and Domestic Violence against Women. (5) The study questionnaire includes the Violence Against Women instrument (henceforth referred to as "VAWI") assessing exposure to psychological, physical and sexual IPV. The VAWI was developed in collaboration with several networks and expert groups and was based partly on the original(6) and revised Conflict Tactics Scales, (7) as well as on work that originated from its critics. (8) Extensive pre-testing and back-translations of the questionnaire were conducted. The prevalence rates from the ten countries included in the multi-country study vary greatly, with life-time estimates ranging between 20-75% for psychological violence, 13-61% for physical violence and 6-59% for sexual violence.(5) Since the Multicountry Study was performed, the VAWI has been used in several more countries. (9)(10)(11)(12)(13)(14) Despite the VAWI's relatively wide use, few peer-reviewed studies have evaluated its psychometric properties. Internal reliability (Cronbach's α) was assessed and confirmed in the

Drop-out analysis
A two-proportion z-test was used to assess statistical significance between the drop-out and the final sample regarding age, country of birth, civil status and the respondents' yearly income before tax. A Bonferroni adjustment to the alpha level was applied.
Comparing those who did not return the questionnaire (n=382) with the final sample of analysis (n=573) revealed that significantly lower response rates were found among nonrespondents who were 18-29 years old, unmarried, foreign born and had low yearly income of 0 -159,999 Swedish Kronor (SEK) before tax. Internal drop-out rates, that is respondents who did not endorse any violence item (n=51), were significantly higher among men who were 18-29 years old, unmarried and had a low yearly income in comparison to the final sample of analysis.
Of those who did not return the questionnaire during the second data collection (n=46), significantly lower response rates were found for women who were unmarried, widowed or divorced.

Assessment instruments: VAWI and NorAQ
The VAWI consists of behaviour-specific items related to psychological (four items), physical (six items) and sexual violence (three items). The physical violence items are further divided into "moderate" (the two first items) and "severe" (the following four items) violence based on the likelihood of physical injury.  NorAQ has been validated in a Swedish context (17) and measures emotional (three items), physical (three items) and sexual (four items) abuse, including different perpetrators, as well as abuse in the health care system. The NorAQ violence items applicable to an intimate partnership ( Figure 1) were included with the intention to compare prevalence rates with those obtained by use of the VAWI. The second sexual violence item was adapted for use in both a male and female population, as the questionnaire constructed for this study was sent to a male population as well (see companion).

Statistical analyses
Principal components analysis (PCA) was conducted to explore the internal construct validity of the violence items. A promax rotation was chosen due to high inter-component correlations (e.g. r=0.49-0.61 for the three dimensions). Decisions on the number of components to extract were based on parallel analysis, Kaiser's eigenvalue-greater-than-one rule, total proportion of variance explained and Cattell's scree plot. This was followed by a pre-determined solution with three components as conceptualized in the VAWI.
The internal reliability of the VAWI was assessed with the Cronbach's alpha for each subscale and for the total violence scale. An alpha of 0.70 or higher was considered satisfactory. (18) Known-groups comparisons were performed to investigate the VAWI's external construct validity. The aim was to see if the instrument was able to differentiate between groups known to differ in exposure to IPV. (19) The following hypotheses were postulated: women who are exposed to physical and/or sexual IPV (life-time exposure, "yes/no") would have poorer self-  (2,(20)(21)(22)(23)(24) and have grown up in a home where they witnessed physical violence between their parents. (9,(25)(26)(27) The Mantel-Haenszel test was used controlling for age, income, civil status, education and country of birth.
Self-perceived health was assessed by "How would you say that your general health has been during the past year?". Response options were dichotomized into "very good/good" and "neither good nor bad/bad/very bad". Childhood exposure to violence was assessed with the question: When you were growing up, did you see your parents (or equivalent) regularly physically hurt one another? ("no" and "yes/unsure").
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.
In addition, life-time prevalence of IPV was compared between the VAWI and the NorAQ and Fisher's exact test was used to test for statistically significant differences at the 95% CI level. Only those respondents who had answered both the VAWI and NorAQ were included (n=77) in this analysis.

Ethical considerations
The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women were followed. (28)  Nearly half of the women had at least three years of university education (n =270; 47.2%) and the mean age was 43 years (SD=13). Of the total sample, 85.1% (n=484) were currently in a relationship (i.e. boyfriend or girlfriend, married, registered partnership or cohabiting), of which the majority were heterosexual relationships (n =566; 98.8%). The rest of the sample was single, widowed or divorced, but had previously been in a relationship (see Table 1).

Internal validity
The Kaiser-Meyer-Olkin measure of sampling adequacy was 0.89 and Bartlett's test of sphericity was significant (p < 0.05), verifying a good fit of the data to the PCA. Parallel analysis, Kaiser's criterion and Cattell's scree test suggested two components (not in Table),

Internal reliability
The Cronbach's alpha coefficient (Table 3) was satisfactory for all subscales in the VAWI conceptual model: 0.79 (psychological scale), 0.80 (physical scale), 0.72 (sexual scale) and 0.88 (total scale). Alpha for the sexual violence scale increased from 0.72 to 0.77 after deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)".

Sexual violence
Demanded to have sex with me even though I did not want to (but did not use physical force) .77 Forced me to have sex against my will by using his/her physical strength (by hitting, holding me firmly or threatening me with a weapon) .64 Forced me to perform sexual acts that I experienced as degrading and/or humiliating .54

External validity
Known-groups comparison As hypothesized, VAWI scores were significantly associated with self-rated health and having witnessed parental (or equivalent) physical violence. Specifically, those who reported exposure to violence also reported worse health and having witnessed parental physical violence to a higher extent.
Comparison of prevalence rates to other studies As assessed with the VAWI, 23 23.6% (n=135), 14.3% (n=82) and 9.2% (n=53; Table 4). Similar 12-month violence exposure rates for physical and sexual violence have been reported in two population-based studiesone in Finland (n=4,464) and one in Sweden (n=4,771) -using comparable methodologies and definitions. (29,30) However, the present study found lower prevalence for physical and sexual violence experienced earlier in life. The aforementioned studies did not report psychological violence. ª Percentage is given in valid percent.

VAWI and NorAQ
Higher prevalence was found by the VAWI compared to NorAQ (see Table 5). However, only the difference for psychological IPV was statistically significant (17.1% vs. 2.6%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (16.9%), for which NorAQ has no corresponding item, and "Belittled and humiliated me in front of other people" (6.5%). Prevalence rates for the two other items on this scale were similar to corresponding items in the NorAQ (see Figure 1).

DISCUSSION
The VAWI subscales of psychological, physical and sexual violence showed good internal consistency. Principal components analysis yielded a two-component solution and a threecomponent solution largely reflected the VAWI's conceptual model. External validity was supported in that the VAWI was able to discriminate between groups known to differ in exposure to physical and/or sexual IPV, that is, respondents with poor vs. good self-rated health and witnessed vs. not witnessed physical violence at home when growing up. Similar past-year prevalence to other Nordic studies was found. Differences in exposure rates of psychological IPV reported in the VAWI and NorAQ exemplify the need for standardized instruments when comparing prevalence of IPV between and within countries.  belonged to other domains than in the conceptual model may reflect that female victims often are not exposed to one form of violence in isolation of the other. (31) For example, the sexual violence item "Forced me to have sex against my will by using his/her physical strength (by hitting, holding me firmly or threatening me with a weapon)" which loaded in both the physical and sexual IPV components is hard to divide into one or the other category.

Internal reliability
The Cronbach's alpha coefficients reported for the subscales in this study are very similar to those found in other studies. (15,16) For example, for all sites combined in the WHO multicountry study, the reliability coefficient was 0.81 for physical and 0.66 for sexual IPV, (15) compared to 0.80 and 0.72 respectively in the current study. These similarities indicate a consistency in the internal reliability of the VAWI across countries despite cultural and socioeconomic differences between the countries.  16 In the current study, deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)" would increase alpha for the sexual violence scale from 0.72 to 0.77. However, given that the current study is explorative and hypothesisgenerating, further studies are needed to determine whether this item needs to be revised or not.

External validity
Known-groups comparison Of the two known-groups used in the comparison, the strongest relationship found in the literature regards exposure to physical and/or sexual IPV and poorer self-rated health. (2,(20)(21)(22)(23)(24) There is also strong evidence that those who are exposed to physical and/or sexual IPV have witnessed their father use physical violence against the mother during childhood. (9,(25)(26)(27) We found support that the combined VAWI subscales of physical and/or sexual violence could discriminate between respondents who had poor vs. good self-rated health and between those who had witnessed vs. not witnessed their parents engaged in physical violence. There is only scant knowledge about how these variables relate to psychological violence, hence these analyses were not deemed appropriate for the purpose of assessing validity.

Comparison of prevalence rates to other studies
Comparisons of our prevalence rates with those in other studies are challenged by differences between methodologies, definitions and reporting styles. Nevertheless, our 12-month violence exposure rates for physical and sexual violence were similar to those reported previously in population-based studies in Finland and Sweden (29,30) using similar definitions and methodologies. However, we found lower rates for earlier-in-life estimates of physical and sexual IPV. The Swedish study found that 28% of women were exposed to physical and 16% were 29% for severe physical and 16% for sexual IPV. These differences are likely due to some minor differences in the definitions between the studies as well as to changes in prevalence rates over time and actual differences between countries. However, they may also owe to an oversight in the questionnaire layout, where the box for ticking violence experienced earlier in life was somewhat unclearly placed. Studies assessing psychological violence in a Nordic context using similar definitions as in the current study could not be found.

VAWI and NorAQ
As the type and number of acts assessed in the VAWI and the NorAQ varied at the outset,

Methodological considerations
As under-reporting is common in surveys assessing IPV, (3,32) estimates of IPV in the current study are probably rather under-than over-estimated. Reasons for under-reporting IPV include forgetting violent acts that took place further back in life, (33) normalizing the violence, blaming the violence on oneself (34) and being fearful of a violent and controlling partner. (35) Moreover, non-responders were over-represented by young and unmarried women, women with lower income and by those born outside of Sweden. Exposure rates to IPV have been found to be especially high in these groups, (21,25) which may further contribute to under-estimated prevalence rates and less robust component solutions in our study.

CONCLUSION
Our analysis indicated that the VAWI has good construct validity and internal reliability in a Swedish context. The results obtained were similar to those reported in the Brazilian study, which implies that the VAWI has good cross-cultural construct validity and internal reliability in an adult female population. However, further studies examining these and other psychometric properties need to be conducted in other countries.

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses, gave input on manuscript drafts. GK designed the project, planned the analyses, gave input on manuscript drafts. All authors read and approved the final manuscript.

FUNDING
This work was supported by a grant from the Swedish Research Council grant number 527-08.

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.

Article focus
• Differences in self-reported exposure to intimate partner violence among women and men have often been found regarding motives for using violence, the context in which the violence occurs and its consequences; however, psychometric properties of instruments assessing intimate partner violence among women and men are seldom investigated in male populations.
• The aim of this study was therefore to examine aspects of the validity and reliability of the Violence Against Women instrument (VAWI) in a randomly selected national population of men

Key messages
• The VAWI conceptual model was only partially replicated and boundaries between psychological, physical and sexual acts of violence were indistinct. This could indicate that different conceptual models, and possibly different assessment instruments, are needed in order to accurately assess men's experiences of IPV in heterosexual relationships.
• Research instruments assessing intimate partner violence need to be validated separately in male and female samples in order to ensure their suitability for the respective groups.
• Theoretical models for understanding men's experiences of violence in heterosexual relationships need to be advanced and should serve to guide in the development and evaluation of gender-specific IPV assessment instruments.

METHODS
Data collection procedures, questionnaires and statistical analyses were the same as those used in the companion paper and are described in greater detail there. A brief description of the statistical analyses specific to the male sample is presented below.  In the second data collection (n=92), response rates were lowest among men who were unmarried, divorced or widowed.

Assessment instruments: VAWI and NorAQ
The VAWI consists of behavior-specific items related to psychological (four items), physical (six items) and sexual IPV (three items). The physical violence items are further divided into "moderate" (the two first items) and "severe" (the following four items) violence based on the likelihood of physical injury. (26) NorAQ was developed to measure abuse in the health care system as well as emotional (three items), physical (three items) and sexual (four items) abuse by different perpetrators (see Figure 1). NorAQ has been shown to have good validity and reliability in a Swedish context.(23, 28)  The internal reliability of the VAWI was assessed with the Cronbach's alpha for each subscale (psychological, physical and sexual violence) and for the total violence scale.

Statistical analyses
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.
Furthermore, life-time prevalence of IPV was compared between the VAWI and the NorAQ.
Only those respondents who had answered both the VAWI and NorAQ were included (n=50) in this analysis. Fisher's exact test (95% CI level) was used to test for differences in prevalence found between the two instruments.

Ethical Considerations
The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women were followed.(29) These and other socio-demographic characteristics of the sample are described in Table 1.

VAWI and NorAQ
A comparison between VAWI and NorAQ was conducted to assess criterion validity (N=50; see Table 5). NorAQ was chosen as it is the only questionnaire measuring violence that has been validated in Sweden in both a male and female (see companion article) population-based sample. The VAWI yielded higher prevalence rates than the NorAQ in relation to all three violence scales. However, only the difference in psychological IPV was statistically significant (30.6% vs. 10.2%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (24%) and "Belittled and humiliated me in front of other people" (16%). Prevalence rates for the other items on this scale were similar to corresponding items in the NorAQ (see Figure 1).

DISCUSSION
The VAWI conceptual model was only partially replicated and boundaries between psychological, physical and sexual acts of violence were indistinct. This finding underlines the importance of investigating psychometric properties of instruments assessing IPV separately for male and female populations. Although the dimensionality of the VAWI was not supported, items composing the three sub-scales, i.e. psychological, physical and sexual violence, showed good internal consistency. Higher prevalence rates for past-year physical and sexual violence were found than those reported in the literature, and than those yielded by a concurrently administered violence questionnaire (NorAQ).

Internal validity
Extraction criteria suggested both a two-and a three-component solution; however, a threecomponent solution was chosen for comparison with the VAWI conceptual model. In general, the VAWI model was not replicated by PCA in the three-component model and other constructs were found which reflected more the severity rather than the types (psychological, physical and sexual) of violence. Although the three-component structure obtained in the female sample (companion article) also reflected the severity of the acts of violence to a certain extent, the PCA structure in that sample conformed better to the VAWI conceptual model of psychological, physical and sexual violence. Another study that conducted exploratory factor analysis on a different instrument assessing psychological and physical IPV among high school students, also found that the boundaries of psychological and physical IPV were indistinct for men whereas they were generally distinct for women. (30) (12,31) although few qualitative studies exist that would have investigated this in depth. Previous research has argued that violent acts are not as fearsome or injury inducing to men as they are to women, and it is indeed possible that men and women are both exposed to and experience IPV in different, gendered ways. (31) This could indicate that different conceptual models, and possibly different assessment instruments, are needed in order to accurately assess men's experiences of IPV in heterosexual relationships. (19) However, further studies, especially qualitative ones, are needed in order to explore this further.

Internal reliability
All three subscales showed acceptable internal reliability. Alpha of the sexual violence scale would augment from .82 to .92 by deletion of the first item ("Demanded to have sex with me even though I did not want to (but did not use physical force)"). However, given that the current study is explorative and hypothesis generating, further studies are needed to assess whether the scale would need to be revised or not.

External validity
Comparison of prevalence rates to other studies

VAWI and NorAQ
The items comprising the VAWI seem to capture a broader spectrum of violent acts, especially psychological violence, than the more systematic types of abuse reflected in the NorAQ. Given the small sample used in this analysis, we cannot draw any conclusions as to which questionnaire is more useful for assessing IPV; however, since they tap a different range of such experiences, the choice of instrument should be made in accordance with the researcher's aim. The overall non-response rate was high (54.6%) and response rates were lower among young men, unmarried men, men with a lower annual income and men born outside Sweden, which compromises the generalizability of our results. Little is known about men's response patterns in surveys on violence exposure perpetrated by their intimate partners. A recent review of gender differences in self-reported IPV cites some studies in which men underreport their experiences of IPV,(6) whereas another review found studies pointing to the contrary.

CONCLUSION
The VAWI conceptual model was only partially replicated and boundaries between psychological, physical and sexual acts of violence were indistinct among men exposed to IPV. This finding suggests that research instruments assessing intimate partner violence need  18 to be validated separately in male and female samples in order to ensure their suitability for the respective groups. However, more and larger studies with better response rates are needed in order to verify the results. Furthermore, theoretical frameworks for understanding men's exposure to intimate partner violence need to be advanced and should serve to guide in the development and evaluation of gender-specific IPV assessment instruments.

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses amd gave input on manuscript drafts. GK designed the project, planned the analyses and gave input on manuscript drafts. All authors read and approved the final manuscript.

FUNDING
This work was supported by a grant from the Swedish Research Council (grant number 527-08).

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.
Consent for sharing the dataset was not obtained but the presented data are anonymised and risk of identification is low.

Psychological Violence
Has your partner systematically and for a longer period tried to repress, degrade or humiliate you? been used in several countries around the world in order to investigate violence against women by their intimate male partners, but aspects of reliability and validity have seldom been investigated.
• The aim of the current study was to explore selected psychometric properties of the VAWI in a randomly selected national sample (n=573) of women.

Key messages
• The current study provides preliminary support for the VAWI subscales of psychological, physical and sexual violence in a Swedish, adult female population.
• This adds to the knowledge of the instrument's cross-cultural validity and reliability, which is of significance when comparing intimate partner violence prevalence rates between countries.

Strengths and limitations
• Cross-sectional study design.
• Further aspects of validity and reliability need to be explored and studies from a diverse range of countries are needed for further cross-cultural assessment.

Keywords
Intimate partner violence * Sweden * Validation * WHO VAW instrument * Psychometric properties * Women The study questionnaire includes the Violence Against Women instrument (henceforth referred to as "VAWI") assessing exposure to psychological, physical and sexual IPV. The VAWI was developed in collaboration with several networks and expert groups and was based partly on the original(6) and revised Conflict Tactics Scales,(7) as well as on work that originated from its critics.(8) Extensive pre-testing, independent back-translations and piloting of the questionnaire were conducted.(9) The prevalence rates from the ten countries included in the multi-country study vary greatly, with life-time estimates ranging between 20-75% for psychological violence, 13-61% for physical violence and 6-59% for sexual violence. (5) Since the Multi-country Study was performed, the VAWI has been used in several more countries. (10)(11)(12)(13)(14)(15) (16) In that study, analyses of data from two female populations, one urban (São Paulo; n=940) and one combined urban and rural area (Zona da Mata; n=1,188) supported the construct validity and internal reliability of the instrument.
The aim of this study was to explore psychometric properties of the VAWI in a randomly selected national sample (n=573) of women aged 18-65 years residing in Sweden. Sweden provides an interesting comparative context due to its linguistic, cultural and socio-economic differences to Brazil.

Procedure, study population and response rate
A sample of 1006 women, aged 18-65 years and residing in Sweden, was randomly selected by Statistics Sweden from the national population register. Data was collected by means of a postal survey between January and March 2009. A requirement for the sample selection was that the respondent was currently or had previously been in an intimate relationship. The response rate was 62.0% (n=624). Women who did not respond to any of the violence items (n=51) were excluded from the analyses, resulting in a total sample of 573 women.

Drop-out analysis
A two-proportion z-test was used to assess statistical significance between the drop-out and the final sample regarding age, country of birth, civil status and the respondents' yearly income before tax. A Bonferroni adjustment to the alpha level was applied.
Comparing those who did not return the questionnaire (n=382) with the final sample of analysis (n=573) revealed that significantly lower response rates were found among nonrespondents who were 18-29 years old, unmarried, foreign born and had low yearly income of 0 -159,999 Swedish Kronor (SEK) before tax. Internal drop-out rates, that is respondents who did not endorse any violence item (n=51), were significantly higher among women who were 18-29 years old, unmarried and had a low yearly income in comparison to the final sample of analysis.
Of those who did not return the questionnaire during the second data collection (n=46), significantly lower response rates were found for women who were unmarried, widowed or divorced.

Assessment instruments: VAWI and NorAQ
The VAWI consists of behaviour-specific items related to psychological (four items), physical (six items) and sexual violence (three items). The physical violence items are further divided into "moderate" (the two first items) and "severe" (the following four items) violence based to differ in exposure to IPV. (19) The following hypotheses were postulated: women who are exposed to physical and/or sexual IPV (life-time exposure, "yes/no") would have poorer selfperceived health (2,(20)(21)(22)(23)(24) and have grown up in a home where they witnessed physical violence between their parents (10,(25)(26)(27). The Mantel-Haenszel test was used controlling for age, income, civil status, education and country of birth. Statistical significance was set at p<0.05.
Self-perceived health was assessed by "How would you say that your general health has been during the past year?". Response options were dichotomized into "very good/good" and "neither good nor bad/bad/very bad". Childhood exposure to violence was assessed with the question: When you were growing up, did you see your parents (or equivalent) regularly physically hurt one another? ("no" and "yes/unsure").
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.
In addition, life-time prevalence of IPV was compared between the VAWI and the NorAQ and Fisher's exact test was used to test for statistically significant differences at the 95% CI level. Only those respondents who had answered both the VAWI and NorAQ were included (n=77) in this analysis. The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women as applicable to a postal survey were followed.(28) For example, a letter was sent to prospective respondents in advance to inform them about the upcoming survey; this provided them with the opportunity to decline the survey before receiving it. Also, although the sampling frame was based on registered individuals, only one survey per household was sent for ethical and safety reasons. Additionally, full anonymity and confidentiality were guaranteed and contact information to a general practitioner (third author on this study), a psychologist and a contact person at Statistics Sweden was provided for additional information and/or referral. The survey was entitled "A study on conflicts, relationships and health". The study description that followed the title stated that the study assesses IPV.

Study population
Nearly half of the women had at least three years of university education (n =270; 47.2%) and the mean age was 43 years (SD=13). Of the total sample, 85.1% (n=484) were currently in a relationship (i.e. boyfriend or girlfriend, married, registered partnership or cohabiting), of which the majority were heterosexual relationships (n =566; 98.8%). The rest of the sample was single, widowed or divorced, but had previously been in a relationship (see Table 1). 0.88 (total scale). Alpha for the sexual violence scale increased from 0.72 to 0.77 after deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)". ª Percentage is given in valid percent.

VAWI and NorAQ
Higher prevalence was found by the VAWI compared to NorAQ (see Table 5). However, only the difference for psychological IPV was statistically significant (17.1% vs. 2.6%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (16.9%), for which NorAQ has no corresponding item, and "Belittled and humiliated me in front of other people" (6.5%). Prevalence rates for the two other items on this scale were similar to corresponding items in the NorAQ (see Appendix 1). belonged to other domains than in the conceptual model may reflect that female victims often are not exposed to one form of violence in isolation of the other.(32) For example, the sexual violence item "Forced me to have sex against my will by using his/her physical strength (by hitting, holding me firmly or threatening me with a weapon)" which loaded in both the physical and sexual IPV components is hard to divide into one or the other category.

Internal reliability
The Cronbach's alpha coefficients reported for the subscales in this study are very similar to those found in other studies. (9,16) For example, for all sites combined in the WHO multicountry study, the reliability coefficient was 0.81 for physical and 0.66 for sexual IPV, (9) compared to 0.80 and 0.72 respectively in the current study. These similarities indicate a consistency in the internal reliability of the VAWI across countries despite cultural and socioeconomic differences between the countries.
In the current study, deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)" would increase alpha for the sexual violence scale from 0.72 to 0.77. However, given that the current study is explorative and hypothesisgenerating, further studies are needed to determine whether this item needs to be revised.

External validity
Known-groups comparison Of the two known-groups used in the comparison, the strongest relationship found in the literature regards exposure to physical and/or sexual IPV and poorer self-rated health. (2,(20)(21)(22)(23)(24) There is also strong evidence that those who are exposed to physical and/or sexual IPV have witnessed their father use physical violence against the mother during childhood. (10,(25)(26)(27) We found support that the combined VAWI subscales of physical and/or sexual violence could discriminate between respondents who had poor vs. good self-rated health and between those who had witnessed vs. not witnessed their parents engage in physical violence. There is only scant knowledge about how these variables relate to psychological violence, hence these analyses were not deemed appropriate for the purpose of assessing validity.

Comparison of prevalence rates to other studies
Comparisons of our prevalence rates with those in other studies are challenged by differences between methodologies, definitions and reporting styles. Nevertheless, our 12-month violence exposure rates for physical and sexual violence were similar to those reported previously in population-based studies in Finland and Sweden (29,30) using similar definitions and methodologies. However, we found lower rates for earlier-in-life estimates of physical and sexual IPV. The Swedish study found that 28% of women were exposed to physical and 16% to sexual violence by a former partner, compared to 14.3% for physical and 9.2% for sexual IPV during the earlier-in-life timeframe in the current study. The figures for the Finnish study were 29% for severe physical and 16% for sexual IPV. These differences are likely due to some minor differences in the definitions between the studies as well as to changes in prevalence rates over time and actual differences between countries. However, they may also owe to an oversight in the questionnaire layout, where the box for ticking violence experienced earlier in life was somewhat unclearly placed. Studies assessing psychological

Methodological considerations
The VAWI was designed for and is primarily used in face-to-face interviews (5)

CONCLUSION
Our analysis indicated that the VAWI has good construct validity and internal reliability in a Swedish context. The results obtained were similar to those reported in the Brazilian study, which implies that the VAWI has good cross-cultural construct validity and internal reliability in an adult female population. However, further studies examining these and other psychometric properties need to be conducted in other countries.

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses and gave input on manuscript drafts.
GK designed the project, planned the analyses and gave input on manuscript drafts. All authors read and approved the final manuscript.

FUNDING
This work was supported by a grant from the Swedish Research Council grant number 527-08.

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.
Consent for sharing the dataset was not obtained but the presented data are anonymised and risk of identification is low.     • The aim of the current study was to explore selected psychometric properties of the VAWI in a randomly selected national population sample (n=573) of women.

Key messages
• The current study provides preliminary support for the VAWI subscales of psychological, physical and sexual violence in a Swedish, adult female population.
• This adds to the knowledge of the instrument's cross-cultural validity and reliability, which is of significance when comparing intimate partner violence prevalence rates between countries.

Strengths and limitations
• Cross-sectional study design.
• Further aspects of validity and reliability need to be explored and studies from a diverse range of countries are needed for further cross-cultural assessment.

Keywords
Intimate partner violence * Sweden * Validation * WHO VAW instrument * Psychometric properties * Women supported the construct validity and internal reliability of the instrument.
The aim of this study was to explore psychometric properties of the VAWI in a randomly selected national population sample (n=573) of women aged 18-65 years residing in Sweden.
Sweden provides an interesting comparative context due to its linguistic, cultural and socioeconomic differences to Brazil.

Drop-out analysis
A two-proportion z-test was used to assess statistical significance between the drop-out and the final sample regarding age, country of birth, civil status and the respondents' yearly income before tax. A Bonferroni adjustment to the alpha level was applied.
Comparing those who did not return the questionnaire (n=382) with the final sample of analysis (n=573) revealed that significantly lower response rates were found among nonrespondents who were 18-29 years old, unmarried, foreign born and had low yearly income of 0 -159,999 Swedish Kronor (SEK) before tax. Internal drop-out rates, that is respondents who did not endorse any violence item (n=51), were significantly higher among men women who were 18-29 years old, unmarried and had a low yearly income in comparison to the final sample of analysis.
Of those who did not return the questionnaire during the second data collection (n=46), significantly lower response rates were found for women who were unmarried, widowed or divorced.

Assessment instruments: VAWI and NorAQ
The VAWI consists of behaviour-specific items related to psychological (four items), physical (six items) and sexual violence (three items). The physical violence items are further divided

Statistical analyses
Principal components analysis (PCA) was conducted to explore the internal construct validity of the violence items. A promax rotation was chosen due to high inter-component correlations  Self-perceived health was assessed by "How would you say that your general health has been during the past year?". Response options were dichotomized into "very good/good" and "neither good nor bad/bad/very bad". Childhood exposure to violence was assessed with the question: When you were growing up, did you see your parents (or equivalent) regularly physically hurt one another? ("no" and "yes/unsure").
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.
In addition, life-time prevalence of IPV was compared between the VAWI and the NorAQ and Fisher's exact test was used to test for statistically significant differences at the 95% CI level. Only those respondents who had answered both the VAWI and NorAQ were included (n=77) in this analysis.

Ethical considerations
The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women as applicable to a postal survey were followed.(28) For example, a letter was sent to prospective respondents in advance to inform them about the upcoming survey; this provided them with the opportunity to decline the survey before receiving it. Also, although the sampling frame was based on registered individuals, only one survey per household was sent for ethical and safety reasons. Additionally, full anonymity and confidentiality were guaranteed and contact information to a general practitioner (third author on this study), a psychologist and a contact person at Statistics Sweden was provided for additional information and/or referral. The survey was entitled "A study on conflicts, relationships and health". The study description that followed the title stated that the study assesses IPV.

Study population
Nearly half of the women had at least three years of university education (n =270; 47.2%) and the mean age was 43 years (SD=13). Of the total sample, 85.1% (n=484) were currently in a relationship (i.e. boyfriend or girlfriend, married, registered partnership or cohabiting), of which the majority were heterosexual relationships (n =566; 98.8%). The rest of the sample was single, widowed or divorced, but had previously been in a relationship (see Table 1).

Internal reliability
The Cronbach's alpha coefficient (Table 3)

VAWI and NorAQ
Higher prevalence was found by the VAWI compared to NorAQ (see Table 5). However, only the difference for psychological IPV was statistically significant (17.1% vs. 2.6%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (16.9%), for which NorAQ has no corresponding item, and "Belittled and humiliated me in front of other people" (6.5%). Prevalence rates for the two other items on this scale were similar to corresponding items in the NorAQ (see Appendix 1).    17 In the current study, deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)" would increase alpha for the sexual violence scale from 0.72 to 0.77. However, given that the current study is explorative and hypothesisgenerating, further studies are needed to determine whether this item needs to be revised or not.

Known-groups comparison
Of the two known-groups used in the comparison, the strongest relationship found in the literature regards exposure to physical and/or sexual IPV and poorer self-rated health. (2,(20)(21)(22)(23)(24) There is also strong evidence that those who are exposed to physical and/or sexual IPV have witnessed their father use physical violence against the mother during childhood. ( were 29% for severe physical and 16% for sexual IPV. These differences are likely due to some minor differences in the definitions between the studies as well as to changes in prevalence rates over time and actual differences between countries. However, they may also owe to an oversight in the questionnaire layout, where the box for ticking violence experienced earlier in life was somewhat unclearly placed. Studies assessing psychological violence in a Nordic context using similar definitions as in the current study could not be found.

VAWI and NorAQ
As the type and number of acts assessed in the VAWI and the NorAQ varied at the outset, some differences in the results from the two instruments were expected.  19 questionnaires when comparing prevalence, as results may vary to a large extent depending on the instrument used.

Methodological considerations
The VAWI was designed for and is primarily used in face-to-face interviews (5)

CONCLUSION
Our analysis indicated that the VAWI has good construct validity and internal reliability in a Swedish context. The results obtained were similar to those reported in the Brazilian study, which implies that the VAWI has good cross-cultural construct validity and internal reliability in an adult female population. However, further studies examining these and other psychometric properties need to be conducted in other countries.

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses and , gave input on manuscript drafts. GK designed the project, planned the analyses and, gave input on manuscript drafts.
All authors read and approved the final manuscript.

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.

Article focus
• The World Health Organization's Violence Against Women instrument (VAWI) has been used in several countries around the world in order to investigate violence against women by their intimate male partners, but aspects of reliability and validity have seldom been investigated.
• The aim of the current study was to explore selected psychometric properties of the VAWI in a randomly selected national sample (n=573) of women.

Key messages
• The current study provides preliminary support for the VAWI subscales of psychological, physical and sexual violence in a Swedish, adult female population.
• This adds to the knowledge of the instrument's cross-cultural validity and reliability, which is of significance when comparing intimate partner violence prevalence rates between countries.

Strengths and limitations
• Cross-sectional study design.
• Further aspects of validity and reliability need to be explored and studies from a diverse range of countries are needed for further cross-cultural assessment.
The aim of this study was to explore psychometric properties of the VAWI in a randomly selected national sample (n=573) of women aged 18-65 years residing in Sweden. Sweden provides an interesting comparative context due to its linguistic, cultural and socio-economic differences to Brazil.

Procedure, study population and response rate
A sample of 1006 women, aged 18-65 years and residing in Sweden, was randomly selected by Statistics Sweden from the national population register. Data was collected by means of a postal survey between January and March 2009. A requirement for the sample selection was that the respondent was currently or had previously been in an intimate relationship. The response rate was 62.0% (n=624). Women who did not respond to any of the violence items (n=51) were excluded from the analyses, resulting in a total sample of 573 women.  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59

Drop-out analysis
A two-proportion z-test was used to assess statistical significance between the drop-out and the final sample regarding age, country of birth, civil status and the respondents' yearly income before tax. A Bonferroni adjustment to the alpha level was applied.
Comparing those who did not return the questionnaire (n=382) with the final sample of analysis (n=573) revealed that significantly lower response rates were found among nonrespondents who were 18-29 years old, unmarried, foreign born and had low yearly income of 0 -159,999 Swedish Kronor (SEK) before tax. Internal drop-out rates, that is respondents who did not endorse any violence item (n=51), were significantly higher among women who were 18-29 years old, unmarried and had a low yearly income in comparison to the final sample of analysis.
Of those who did not return the questionnaire during the second data collection (n=46), significantly lower response rates were found for women who were unmarried, widowed or divorced.
Self-perceived health was assessed by "How would you say that your general health has been during the past year?". Response options were dichotomized into "very good/good" and "neither good nor bad/bad/very bad". Childhood exposure to violence was assessed with the question: When you were growing up, did you see your parents (or equivalent) regularly physically hurt one another? ("no" and "yes/unsure").
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.
In addition, life-time prevalence of IPV was compared between the VAWI and the NorAQ and Fisher's exact test was used to test for statistically significant differences at the 95% CI level. Only those respondents who had answered both the VAWI and NorAQ were included (n=77) in this analysis.  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women as applicable to a postal survey were followed.(28) For example, a letter was sent to prospective respondents in advance to inform them about the upcoming survey; this provided them with the opportunity to decline the survey before receiving it. Also, although the sampling frame was based on registered individuals, only one survey per household was sent for ethical and safety reasons. Additionally, full anonymity and confidentiality were guaranteed and contact information to a general practitioner (third author on this study), a psychologist and a contact person at Statistics Sweden was provided for additional information and/or referral. The survey was entitled "A study on conflicts, relationships and health". The study description that followed the title stated that the study assesses IPV.

Internal reliability
The Cronbach's alpha coefficient (Table 3)

VAWI and NorAQ
Higher prevalence was found by the VAWI compared to NorAQ (see Table 5). However, only the difference for psychological IPV was statistically significant (17.1% vs. 2.6%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (16.9%), for which NorAQ has no corresponding item, and "Belittled and humiliated me in front of other people" (6.5%). Prevalence rates for the two other items on this scale were similar to corresponding items in the NorAQ (see Appendix 1). belonged to other domains than in the conceptual model may reflect that female victims often are not exposed to one form of violence in isolation of the other.(32) For example, the sexual violence item "Forced me to have sex against my will by using his/her physical strength (by hitting, holding me firmly or threatening me with a weapon)" which loaded in both the physical and sexual IPV components is hard to divide into one or the other category.

Internal reliability
The Cronbach's alpha coefficients reported for the subscales in this study are very similar to those found in other studies. (9,16) For example, for all sites combined in the WHO multicountry study, the reliability coefficient was 0.81 for physical and 0.66 for sexual IPV, (9) compared to 0.80 and 0.72 respectively in the current study. These similarities indicate a consistency in the internal reliability of the VAWI across countries despite cultural and socioeconomic differences between the countries.
In the current study, deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)" would increase alpha for the sexual violence scale from 0.72 to 0.77. However, given that the current study is explorative and hypothesisgenerating, further studies are needed to determine whether this item needs to be revised.

External validity
Known-groups comparison Of the two known-groups used in the comparison, the strongest relationship found in the literature regards exposure to physical and/or sexual IPV and poorer self-rated health. (2,(20)(21)(22)(23)(24) There is also strong evidence that those who are exposed to physical and/or sexual IPV have witnessed their father use physical violence against the mother during childhood. (10,(25)(26)(27) We found support that the combined VAWI subscales of physical and/or sexual violence could discriminate between respondents who had poor vs. good self-rated health and between those who had witnessed vs. not witnessed their parents engage in physical violence. There is only scant knowledge about how these variables relate to psychological violence, hence these analyses were not deemed appropriate for the purpose of assessing validity.

Comparison of prevalence rates to other studies
Comparisons of our prevalence rates with those in other studies are challenged by differences between methodologies, definitions and reporting styles. Nevertheless, our 12-month violence exposure rates for physical and sexual violence were similar to those reported previously in population-based studies in Finland and Sweden (29,30) using similar definitions and methodologies. However, we found lower rates for earlier-in-life estimates of physical and sexual IPV. The Swedish study found that 28% of women were exposed to physical and 16% to sexual violence by a former partner, compared to 14.3% for physical and 9.2% for sexual IPV during the earlier-in-life timeframe in the current study. The figures for the Finnish study were 29% for severe physical and 16% for sexual IPV. These differences are likely due to some minor differences in the definitions between the studies as well as to changes in prevalence rates over time and actual differences between countries. However, they may also owe to an oversight in the questionnaire layout, where the box for ticking violence experienced earlier in life was somewhat unclearly placed. Studies assessing psychological

Methodological considerations
The VAWI was designed for and is primarily used in face-to-face interviews (5)

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses and gave input on manuscript drafts.
GK designed the project, planned the analyses and gave input on manuscript drafts. All authors read and approved the final manuscript.

FUNDING
This work was supported by a grant from the Swedish Research Council grant number 527-08.

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.

Key messages
• The current study provides preliminary support for the VAWI subscales of psychological, physical and sexual violence in a Swedish, adult female population.
• This adds to the knowledge of the instrument's cross-cultural validity and reliability, which is of significance when comparing intimate partner violence prevalence rates between countries.

Strengths and limitations
• Cross-sectional study design.
• Further aspects of validity and reliability need to be explored and studies from a diverse range of countries are needed for further cross-cultural assessment.
The aim of this study was to explore psychometric properties of the VAWI in a randomly selected national population sample (n=573) of women aged 18-65 years residing in Sweden.

Drop-out analysis
A two-proportion z-test was used to assess statistical significance between the drop-out and the final sample regarding age, country of birth, civil status and the respondents' yearly income before tax. A Bonferroni adjustment to the alpha level was applied.
Comparing those who did not return the questionnaire (n=382) with the final sample of analysis (n=573) revealed that significantly lower response rates were found among nonrespondents who were 18-29 years old, unmarried, foreign born and had low yearly income of 0 -159,999 Swedish Kronor (SEK) before tax. Internal drop-out rates, that is respondents who did not endorse any violence item (n=51), were significantly higher among men women who were 18-29 years old, unmarried and had a low yearly income in comparison to the final sample of analysis.
Of those who did not return the questionnaire during the second data collection (n=46), significantly lower response rates were found for women who were unmarried, widowed or divorced.

Assessment instruments: VAWI and NorAQ
The VAWI consists of behaviour-specific items related to psychological (four items), physical (six items) and sexual violence (three items). The physical violence items are further divided  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  NorAQ has been validated in a Swedish context (17) and measures emotional (three items), physical (three items) and sexual (four items) abuse, including different perpetrators, as well as abuse in the health care system. The NorAQ violence items applicable to an intimate partnership (Appendix 1) were included with the intention to compare prevalence rates with those obtained by use of the VAWI. The second sexual violence item was adapted for use in both a male and female population, as the questionnaire constructed for this study was sent to a male population as well (see companion paper entitled 'Psychometric properties of the WHO Violence Against Women instrument in a male population-based sample in Sweden').
Prevalence of psychological, physical and sexual violence was calculated for the past year and for earlier in life, for comparisons with prevalence rates presented in other studies.

Ethical considerations
The Regional Ethics Review Board located in Gothenburg gave approval for this study (Dnr: 527-08) and the WHO ethical and safety recommendations for research on domestic violence against women as applicable to a postal survey were followed.(28) For example, a letter was sent to prospective respondents in advance to inform them about the upcoming survey; this provided them with the opportunity to decline the survey before receiving it. Also, although the sampling frame was based on registered individuals, only one survey per household was sent for ethical and safety reasons. Additionally, full anonymity and confidentiality were guaranteed and contact information to a general practitioner (third author on this study), a psychologist and a contact person at Statistics Sweden was provided for additional information and/or referral. The survey was entitled "A study on conflicts, relationships and health". The study description that followed the title stated that the study assesses IPV.

VAWI and NorAQ
Higher prevalence was found by the VAWI compared to NorAQ (see Table 5). However, only the difference for psychological IPV was statistically significant (17.1% vs. 2.6%; p<0.05). This difference owed principally to the VAWI items "Insulted me in a way that made me feel bad about myself" (16.9%), for which NorAQ has no corresponding item, and "Belittled and humiliated me in front of other people" (6.5%). Prevalence rates for the two other items on this scale were similar to corresponding items in the NorAQ (see Appendix 1).

DISCUSSION
The VAWI subscales of psychological, physical and sexual violence showed good internal consistency. Principal components analysis yielded a two-component solution and a threecomponent solution largely reflected the VAWI's conceptual model. External validity was supported in that the VAWI was able to discriminate between groups known to differ in exposure to physical and/or sexual IPV, that is, respondents with poor vs. good self-rated health and witnessed vs. not witnessed physical violence at home when growing up. Similar past-year prevalence to other Nordic studies was found. Differences in exposure rates of psychological IPV reported in the VAWI and NorAQ exemplify the need for standardized instruments when comparing prevalence of IPV between and within countries.  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59 17 In the current study, deleting the item "Demanded to have sex with me even though I did not want to (but did not use physical force)" would increase alpha for the sexual violence scale from 0.72 to 0.77. However, given that the current study is explorative and hypothesisgenerating, further studies are needed to determine whether this item needs to be revised or not.

Known-groups comparison
Of the two known-groups used in the comparison, the strongest relationship found in the literature regards exposure to physical and/or sexual IPV and poorer self-rated health. (2,(20)(21)(22)(23)(24) There is also strong evidence that those who are exposed to physical and/or sexual IPV have witnessed their father use physical violence against the mother during childhood. (10,(25)(26)(27)) (9,(25)(26)(27) We found support that the combined VAWI subscales of physical and/or sexual violence could discriminate between respondents who had poor vs. good self-rated health and between those who had witnessed vs. not witnessed their parents engaged in physical violence. There is only scant knowledge about how these variables relate to psychological violence, hence these analyses were not deemed appropriate for the purpose of assessing validity.

Comparison of prevalence rates to other studies
Comparisons of our prevalence rates with those in other studies are challenged by differences between methodologies, definitions and reporting styles. Nevertheless, our 12-month violence exposure rates for physical and sexual violence were similar to those reported previously in population-based studies in Finland and Sweden (29,30) using similar definitions and methodologies. However, we found lower rates for earlier-in-life estimates of physical and  18 sexual IPV. The Swedish study found that 28% of women were exposed to physical and 16% to sexual violence by a former partner, compared to 14.3% for physical and 9.2% for sexual IPV during the earlier-in-life timeframe in the current study. The figures for the Finnish study were 29% for severe physical and 16% for sexual IPV. These differences are likely due to some minor differences in the definitions between the studies as well as to changes in prevalence rates over time and actual differences between countries. However, they may also owe to an oversight in the questionnaire layout, where the box for ticking violence experienced earlier in life was somewhat unclearly placed. Studies assessing psychological violence in a Nordic context using similar definitions as in the current study could not be found.

Methodological considerations
The VAWI was designed for and is primarily used in face-to-face interviews(5), whereas the current study administered the VAWI via a postal survey. The implications of different modes of data collection are difficult to assess due to multiple influencing factors, including the method of initial contact with the respondents, visual versus oral presentation of response choices, method of sampling as well as differing cultural and social contexts.(33) Previous studies have found disclosure of sensitive topics to be higher in self-administered modes compared to face-to-face interviews,(33) also when assessing IPV (34). However, there is a scarcity of experimental or randomized study designs comparing different modes of data collection. (33) Nonetheless, the main known limitation of postal surveys is lowered response rates.(34) The current study included two reminders in an effort to minimize drop-out rates. Non-responders were over-represented by young and unmarried women, women with low annual income and by those born outside of Sweden. Exposure rates of IPV have been found to be especially high in these groups, (21,25) which may further contribute to under-estimated prevalence rates and less robust component solutions in our study. Furthermore, the earlier-in-life estimates may have been underestimated due to a minor detail on the questionnaire lay-out. In addition, theAs under-reporting is common in surveys assessing IPV, (3,35) 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  NorAQ is small, which limits our ability to draw conclusions or generalize to the target population. Moreover, non-responders were over-represented by young and unmarried women, women with lower income and by those born outside of Sweden. Exposure rates to IPV have been found to be especially high in these groups, (21,25) which may further contribute to under-estimated prevalence rates and less robust component solutions in our study.

CONCLUSION
Our analysis indicated that the VAWI has good construct validity and internal reliability in a Swedish context. The results obtained were similar to those reported in the Brazilian study, which implies that the VAWI has good cross-cultural construct validity and internal reliability in an adult female population. However, further studies examining these and other psychometric properties need to be conducted in other countries.

COMPETING INTERESTS
The authors declare that they have no competing interests.

CONTRIBUTORSHIP STATEMENT
LN conducted all analyses, wrote the first draft of the manuscript and rewrote new drafts based on input from co-authors. CT planned the analyses and , gave input on manuscript drafts. GK designed the project, planned the analyses and, gave input on manuscript drafts.
All authors read and approved the final manuscript.

DATA SHARING STATEMENT
The dataset is available form the corresponding author at lotta.nybergh@socmed.gu.se.
Consent for sharing the dataset was not obtained but the presented data are anonymised and risk of identification is low.