Brian Short

Intraclass correlation among measures related to tobacco use by adolescents: estimates, correlates, and applications in intervention studies.

Tobacco intervention studies that employ a community trial design require adjustment to the usual analytic methods to account for the allocation of intact social groups to study conditions and the positive intraclass correlation (p) that is inevitable in such a design. In the absence of valid estimates of the relevant p, investigators seeking to establish an appropriate sample size could only guess about the magnitude of the problem. We recently published estimates of p for common measures of adolescent tobacco use, but those estimates were unadjusted for potential covariates and so represented an upper limit on the magnitude of p. This report demonstrates how estimates of intraclass correlation may be substantially reduced through regression adjustment for easily measured covariates. Results show that both the p and the residual variance can be reduced, by an average of 20 and 11%, respectively, offering greater efficiency for investigators who plan future studies and who are able to measure those covariates in their studies. Future work should seek both to replicate this work and to extend it; for example, to cohort designs where the improvements might be even greater.

Intraclass Correlation among Measures Related to Alcohol Use by School Aged Adolescents: Estimates, Correlates and Applications in Intervention Studies

School-based alcohol use prevention studies frequently employ designs in which schools are assigned to treatment conditions while observations are made on individuals. The nesting of schools within treatment conditions requires that the treatment effect be assessed against the between-school variance; unfortunately, that variance is usually larger than for randomly constituted groups and its precision is usually less than that for the within-school variance. These factors often combine to reduce power substantially. To address these problems, investigators need good estimates for the intraclass correlation which together with the number of observations per school determines the magnitude of the extra variation in the nested design. This article presents estimates of school-level intraclass correlation for measures related to alcohol use among ninth and twelfth grade students and discusses their use in planning new studies and analyzing previous or current studies.

Relationships among Alcohol Availability, Drinking Location, Alcohol Consumption, and Drinking Problems in Adolescents

We examined relationships among perceived alcohol availability, drinking location, alcohol consumption, and drinking problems. Subjects were 3,372 adolescent drinkers, ages 16-18, who participated in the Communities Mobilizing for Change on Alcohol Project baseline survey. Mixed-model regression was employed to identify predictors of alcohol consumption and drinking consequences. Perceived alcohol availability was significantly associated with higher levels of alcohol consumption for males. Drinking in a public location with higher levels of alcohol consumption for females. Results underscore the importance of youth alcohol assessibility.

Environmental Predictors of Drinking and Drinking-Related Problems in Young Adults

We examined relationships among drinking norms, peer alcohol use, alcohol availability, drinking location, alcohol consumption, and drinking-related problems among young adult drinkers. The specific objectives of our study were to assess the relative contribution of normative and physical environmental factors to drinking and drinking consequences. Subjects were 3,095 young adults, aged eighteen to twenty years old who participated in the Communities Mobilizing for Change on Alcohol Project baseline survey. Alcohol consumption (i.e., number of drinks on the last occasion) and drinking consequences served as dependent variables. Multiple linear regression was used to identify predictors of alcohol consumption, and logistic regression was employed to identify predictors of drinking consequences. Drinking norms and peer alcohol use were positively related to alcohol consumption and to drinking consequences. Drinking in a public setting was positively related to alcohol consumption, but not to drinking-related problems. Findings suggest that policies and programs that alter the normative and physical environment surrounding drinking may reduce alcohol consumption and subsequent problems in young adults.

Measurement of alcohol and tobacco availability to underage students

Communities Mobilizing for Change on Alcohol (CMCA) and Tobacco Policy Options for Prevention (TPOP) are randomized multicommunity trials designed to reduce the availability of alcohol (CMCA) and tobacco (TPOP) products to underage youth. We assessed the test-retest reliability and internal consistency of perceived availability measures developed for surveys of middle and high school students. Two questionnaires were administered twice, each to separate student populations (CMCA: 9th graders; TPOP; 8th graders) 21 to 26 days apart. A total of 111 (CMCA) and 70 (TPOP) subjects participated at both times (participation rate of 93.3% and 84.3%, respectively). There were no statistically significant differences in the distribution of demographic variables, alcohol or tobacco use variables, or availability variables across administrations. The scales assessing perceived access to alcohol or tobacco had high internal consistency (Cronbachs alpha of 0.84 for alcohol and 0.77 for tobacco), but moderate to low test-retest correlations (0.58 and 0.13, respectively). Single-item measures of sources of alcohol and tobacco and reported buy attempts also had moderate to low test-retest correlates (0.10-0.65). We recommend that items assessing tobacco and alcohol availability to underage youth be further developed.