Reporting effect sizes in scientific articles is increasingly widespread and encouraged by journals; however, choosing an effect size for analyses such as mixed-effects regression modeling and hierarchical linear modeling can be difficult. One relatively uncommon, but very informative, standardized measure of effect size is Cohen’s f2, which allows an evaluation of local effect size, i.e., one variable’s effect size within the context of a multivariate regression model. Unfortunately, this measure is often not readily accessible from commonly used software for repeated-measures or hierarchical data analysis. In this guide, we illustrate how to extract Cohen’s f2 for two variables within a mixed-effects regression model using PROC MIXED in SAS® software. Two examples of calculating Cohen’s f2 for different research questions are shown, using data from a longitudinal cohort study of smoking development in adolescents. This tutorial is designed to facilitate the calculation and reporting of effect sizes for single variables within mixed-effects multiple regression models, and is relevant for analyses of repeated-measures or hierarchical/multilevel data that are common in experimental psychology, observational research, and clinical or intervention studies.

This study used semi-parametric group-based modeling to explore unconditional and conditional trajectories of self-reported depressed mood from age 12 to 25. Drawing on data from the National Longitudinal Study of Adolescent Health (N=11,559), four distinct trajectories were identified: no depressed mood, stable low depressed mood, early high declining depressed mood, and late escalating depressed mood. Baseline risk factors associated with greater likelihood of membership in depressed mood trajectory groups compared to the no depressed mood group included being female, Black/African American, Hispanic/Latino American, or Asian American, lower SES, using alcohol, tobacco, or other drugs on a weekly basis, and delinquent behavior. Baseline protective factors associated with greater likelihood of membership in the no depressed mood group compared to the depressed mood trajectory groups included two-parent family structure, feeling connected to parents, peers, or school, and self esteem. With the exception of delinquent behavior, risk and protective factors also distinguished the likelihood of membership among several of the three depressed mood groups. The results add to basic etiologic research regarding developmental pathways of depressed mood in adolescence and young adulthood.

The current study examined the natural history of smoking from adolescence to adulthood in a community sample. Participants were from a longitudinal study (N = 4,035, 51.7% female, average age = 29 years). Group-level analyses showed a significant increase in smoking from adolescence to young adulthood and a nonsignificant decline after the mid-20s. Individual-level analyses showed that there was appreciable cessation and relapse but little new initiation in adulthood. Both adolescent and young adult smoking status were powerful predictors of adult smoking. Moreover, there was less cessation among less educated individuals and those with smoking parents, and more cessation among those who assumed adult social roles. The findings support the importance of prevention campaigns aimed at adolescent smoking and also suggest that those with lower educational attainment or with a family history of smoking are at heightened risk.

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A longitudinal multigenerational design was used to examine the intergenerational transmission of smoking and the correlated transmission of parental support and control. Whether maternal socialization of adolescent smoking (both general parenting practices and smoking-specific strategies) would predict adolescent smoking both directly and indirectly by affecting peer affiliations was tested. There was strong evidence for the intergenerational transmission of cigarette smoking and for the relation between peer smoking and adolescent smoking. Both general parenting practices and smoking-specific discussion and punishment were significantly related to adolescents' smoking, especially for adolescent-reported parenting. Support for the intergenerational transmission of parenting practices emerged only in mothers' reports of support. Results suggest expanding current peer-focused prevention efforts to include parental socialization strategies.