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Validation of the Internal Mental Distress Scale (IMDS) to the Rasch Measurem ent Model, GAIN Methods Report 1.2

Purpose. The purpose of this report is to provide a brief psychometric analysis of the Internal Mental Distress Scale using the Rasch measurement model. The 43-item IMDS is a count of past-year symptoms related to internalizing disorders, including somatic, anxiety, depression, traumatic stress and suicide thoughts. It is based on the DSM-IV-TR, the Hopkins Symptom Checklist 90 (HSCL-90), the Mississippi Scale of Post Traumatic Stress Disorder (PTSD) and common screening items for suicide risk (homicidal thoughts, suicidal thoughts, plans, means, attempts). Methods. Data were analyzed on 7,435 persons who presented for substance problem screening. Rasch analysis included an examination of: person and item reliabilities; construct validity including item and person fit statistics; and differential item functioning (DIF) across subgroups. DIF analysis allowed us to determine if the relative item estimates (i.e., item difficulty estimates) remained invariant across subgroups of persons. Results. The IMDS performs well as a measure of the construct of internal mental distress. Both items and scales form the theoretically expected hierarchies with a person internal consistency reliability of .89 and an item reliability of 1.00. The persons’ responses generally conformed to the expectations of the Rasch model. Of the 43 items in the IMDS, significant DIF (i.e., > .5 SD = .58 logits) occurred in 3 items for males vs. females, 11 items for youth vs. adults, 4 items for race when using African American as the reference group, and 17 items for primary substances when using alcohol as the referent. In terms of person fit, over 86% of the persons exhibited person infit and outfit that were low or moderate (LMI/LMO) and were thus regarded as fitting the Rasch model expectations well from a clinical perspective. The group termed, Atypical Type 1, (10 %) consisted of persons with low/moderate infit and high outfit (LMI/HO), where the overall score may underestimate severity since these tended to be people who were having suicidal thoughts (higher risk items) but were unexpectedly low on depression, about moderate on somatic, low to moderate on anxiety, except for the three highest anxiety items where they were higher, and low to moderate on trauma (which all tended to be lower risk than suicide). The HI /LMO group, Atypical Type 2, consisted of only 11 people (.1%). This group was characterized by low depression but was especially high on trauma, stress, and anxiety while being somewhat higher on somatic complaints and suicidal ideation. The HI/HO group, Atypical Type 3, (3.4%) tended to be valid high scorers who also had an overall measure that may underestimate severity somewhat and tended to have higher anxiety, higher trauma and higher suicidal ideation while their depression and somatic symptoms were moderate. These persons misfit because they strongly endorsed most of the higher risk items but only endorsed some of the lower risk items at moderate levels. Conclusion. The IMDS functioned well as a unidimensional measure with good person and item reliability. In terms of item quality, there were no items with both infit and outfit values outside of our criterion of .75-1.33. While there were several items with high outfit values, these indicated that a few people endorsed these items