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Collectivity of drinking behavior among adolescents: An analysis of the Norwegian ESPAD data 1995-2011

Aims The aim of the current study was to test empirically two predictions from Skog's theory of collectivity of drinking behavior, using time series data from Norwegian adolescents. The two specific predictions were: 1) A change in mean alcohol consumption is positively associated with a change in the proportion of heavy drinkers, and 2) A change in mean alcohol consumption is positively associated with a change at all consumption levels. Data & Methods The present analyses are based on ESPAD data collected from Norwegian adolescents (15-16 years) in 1995, 1999, 2003, 2007 and 2011. The relationship between mean consumption and the proportion of heavy drinkers was analyzed by regressing the proportions of heavy drinkers at each time point on the consumption means at each time point. In order to assess whether adolescents at all consumption levels, from light to heavy drinkers, changed collectively as mean consumption changed, we regressed log-transformed consumption means on the log-transformed percentile values (P25, P50, P75, P90 and P95). The analysis was restricted to adolescents who had consumed alcohol in the last 30 days (total n = 7554). Results The results showed a strong relationship between mean alcohol consumption and the proportion of heavy drinkers. An increase in mean consumption was also associated with an increase at all consumption levels, from light to heavy drinkers. Conclusion The results of the current study are in line with the theory of collectivity of drinking behavior. The findings of this study suggest that by reducing the total consumption of alcohol among adolescents, consumption and risk of harm may be reduced in all consumer groups.


Introduction
The European School Survey Project on Alcohol and Other Drugs (ESPAD) does not only provide a unique opportunity to compare adolescent substance use across a large number of countries, but with its regular data collection every fourth year it has also provided important time series of such data. This study is based on the latter. In Norway, per capita alcohol sales increased by 50 per cent from 1993 to 2008, and thereafter a slight decrease was observed (Edland-Gryt, Bryhni, Skretting, Lund, & Bye, 2012). Among young people, however, the increase in consumption during the 1990s peaked around the first years of the new millennium, and thereafter it decreased (Bye, 2012;Vedøy & Skretting, 2009). Similar diverging trends in adult and youth drinking have been observed in Sweden (CAN, 2012) and the United Kingdom (Meier, 2010). These diverging trends in alcohol consumption may appear contradictory to Skog's theory of collectivity of drinking behaviour (Skog, 1985), but as we shall see this is not necessarily the case.
However, a recent study of Swedish youth (Hallgren, Leifman, & Andréasson, 2012) suggested that the observed decrease in youth drinking resulted in a polarization of youth drinkers. While consumption had decreased in most consumer groups, it had increased in the heaviest drinking groups, which is inconsistent with Skog's theory.
In light of this, the current study investigated the applicability of Skog's theory to the recent trends in youth drinking in Norway.
According to Skog's theory of the distribution of alcohol consumption, drinking is a social behavior, and individual drinking tends to be modified by interaction with other individuals (Skog, 1985(Skog, , 1991a(Skog, , 1991b. Since the individuals in one's social network in turn interact with other individuals who interact with yet more individuals, there exists a complex network where each individual is tied to the drinking culture as a whole. The result is a collectivity of drinking behavior where all groups of consumers, ranging from light to heavy drinkers, tend to change in the same direction when total consumption changes, that is they move 'in concert up and down the scale of consumption'.
The distribution of alcohol consumption is heavily skewed and the majority of drinkers consumes less than the population mean, while a minority of heavy drinkers consumes most of the alcohol (Skog, 1985(Skog, , 1991a. Given that all drinking groups, from light drinkers to heavy drinkers, change with a change in the total consumption, one implication of this collectivity is that there must be a close relationship between the mean consumption and the proportion of heavy drinkers in the population. A further implication of Skog's theory for alcohol policy is that effective efforts to reduce the total consumption of alcohol will cause a reduction in heavy drinking and alcohol related harm. Despite widespread recognition of Skog's theory and its importance for public alcohol policies, as well as criticism of its empirical testability (Gmel & Rehm, 2000), surprisingly little research has been conducted that tests its predictions (Gmel & Rehm, 2000;Rossow & Clausen, 2013;Rossow, Mäkelä, & Kerr, 2014). The assumption that all drinkers 'move in concert' when mean consumption changes, has been tested by analyzing the association between the mean consumption and the consumption at various drinking levels (e.g. light, moderate and heavy drinkers). Skog's own empirical analyses of such collective changes were mainly based on regional variations, whereas the interpretation is usually related to temporal changes. Skog (1985) noted this as a weakness that needed to be addressed in future studies. Although such studies have been warranted also in the succeeding years (Gmel & Rehm, 2000;Skog, 2001), they are still quite few. Very recent exceptions are studies of the adult population in a several countries. Two studies from Sweden demonstrated that consumption increased in all consumer groups between 1996/1997and 2004/2005(Tryggvesson, 2013 (Rossow et al., 2014). Now, turning to the youth population, the observation that alcohol consumption among adolescents has declined, whereas the prevalence of heavy drinkers may have increased, has led several authors to suggest that a "polarization" between light and heavy drinkers has taken place (Bjarnason, 2006;Livingston, 2008;Meier, 2010), rather than collective changes.
Two recent studies among Swedish youth that have examined changes in mean consumption and consumption among heavy drinkers over time have produced different findings; one apparently contradicting Skog's theory (Hallgren et al., 2012), whereas the other provided evidence that was in line with Skog's theory (Norström & Svensson, 2014).
The aim of the current study was to test empirically two predictions of collective changes from Skog's theory, using time series data for Norwegian adolescents. The two specific predictions were: 1) A change in mean alcohol consumption is positively associated with a change in the proportion of heavy drinkers, and 2) A change in mean alcohol consumption is positively associated with a change at all consumption levels.

Samples
The present analyses are based on Norwegian data from the ESPAD study, which was conducted five times: In 1995In , 1999In , 2003

Measures
An estimate of past 30 days drinking frequency was obtained by using responses to the question "On how many occasions (if any) have you had any alcohol beverage to drink during the last 30 days?" The response alternatives were: "0", "1-2", "3-5", "6-9", "10-19", "20-39" and "40 or more". The responses were recoded to number of days per month (0, 1.5, 4, 7.5, 14.5, 30 and 30 respectively). The response category "40 or more" was coded 30 because it is the maximum possible number of days in a 30 day period.
Responses from five beverage specific questions were used to determine the amount of alcohol consumed at the last drinking occasion. The question was worded: "If you drank beer/wine/spirits/ alcopops/cider that last day you drank any alcohol, how much did you drink?" The response alternatives for beer, alcopops and cider were "I never drink beer/alcopops/cider" (coded 0), "I did not drink beer/alcopops/cider on the last day that I drank alcohol" (coded 0), "<50 cl" (coded 25), "50-100 cl" (coded 66), "101-200 cl" (coded 150), and ">200 cl" (coded 200).
The response alternatives for wine in response alternatives in 2003and 2011. In 1995and 1999 the response alternatives were "I never drink wine" (coded 0), "I did not drink wine on the last day that I drank alcohol" (coded 0), "Less than a glass (<10 cl)" (coded 5), "1-2 glasses To obtain results comparable to previous research, we investigated whether change in mean alcohol consumption was associated with change at all consumption levels by applying the same method as Skog (1985

Results
Mean alcohol consumption increased from 1995 to 2003 and decreased from 2003 to 2011 (see Table 1). This was evident for  Table 2 shows that an increase in mean consumption was strongly associated with an increase at all consumption levels. More specifically, a 1% increase in mean consumption was associated with 1.33%, 0.8%, 1%, 1% and 1.17% increase in the 25, 50, 75, 90 and 95 percentile values, respectively. Figure 1 shows the percentile values by mean consumption in the 10 gender specific samples and further illustrates this systematic pattern in the distribution of consumption.

Sensitivity analyses
Several approaches were used to test the robustness of our findings. First, we tested

Discussion
This study is one of few that have ad- ing consumption (Knibbe et al., 1985;Lemmens et al., 1990), whereas this was not the case in the Swedish study of youth alcohol consumption addressing a period with an overall decreasing trend in consumption (Hallgren et al., 2012). The present study found support for a collective  (Skog, 2001;Skog & Rossow, 2006 (Skog & Rossow, 2006). Panel data from Finland in 1968 and1969 showed, however, that also at the individual level alcohol consumption increased in all consumer groups when the mean consumption increased markedly (Mäkelä, 2002).

Strengths and limitations
The data comprised five representative cross sections of youths over sixteen years, which allowed investigation of trends in alcohol consumption over time, as well as investigation of differences in trends in sub-groups of drinkers. Overall, the response rates were quite high compared to other surveys due to anonymity and the survey administration taking place in school classrooms.The findings were robust, as repeated analyses applying different consumption estimates and different cut-off points for heavy drinkers, did not alter the findings much.
There are, however, several limitations that should be noted. Firstly, alcohol consumption was self-reported, which can lead to under-reporting (Midanik, 1982;Ramstedt, 2010 (Romelsjö & Branting, 2000), selective non-response would deflate the mean, the variance and reduce skewness.
If this is the case, low response rates will result in underestimation of the strength of the relationship between the mean and the proportion of heavy drinkers, and the strength of the relationship between the mean and the percentile values (especially higher percentiles).
The use of the "last occasion method" to measure volume of alcohol consumption may not be ideal, but no thorough investigation of its performance is currently available. The quantity consumed at the last drinking occasion can be different from the individual's typical quantity. Whether this results in biased estimates of volume of consumption, and the direction of such bias, is unknown. It is also unknown whether the size of the bias will be related to level of consumption. Adolescents with a high volume of consumption will have a higher drinking frequency and probably also greater variance in the quantity they consume when drinking. It is therefore more likely that a high consumer (compared to a moderate or low consumer) will report from an occasion where an unusual quantity was consumed. Whether this leads to inflated or deflated estimates is not known, but it is not impossible that over-and underestimation of quantity will cancel each other out. Moreover, as most of the variation in volume of consumption will be due to differences in drinking frequency, we believe that the difference between the "last drinking occasion method" and other methods will be small.

Implications
According to recent ESPAD data around 80 percent of students in Europe had used alcohol in the past 12 months in 2011 (Hibell et al., 2012). As young people are particularly susceptible to alcohol related harm, the importance of prevention is substantial (Toumbourou et al., 2007). The findings of this study suggest that by reducing the total consumption of alcohol among young people, consumption and risk of harm may be reduced in all consumer groups. Effective measures of relevance in this respect include high alcohol taxes (Chaloupka, Grossman, & Saffer, 2002;Wagenaar, Salois, & Komro, 2009) and restrictions on availability of alcohol, e.g. minimum legal age for purchase (Babor et al., 2010;Wagenaar & Toomey, 2002).
The scarcity of studies applying temporal data to test Skog's theory of collectivity represents a challenge to be met in future studies. Ideally, these should also include panel data to assess both collective and individual changes, and they should also include data from drinking cultures in other parts of the world where alcohol is less integrated in society.
Declaration of interest None.