As the dependent variable is continuous, an effect size in the d family of standardized mean differences might be considered first. This effect size measure has been used elsewhere in the context of multivariate mixed-effects regression models using repeated measures of subjects, for example mean differences in an outcome across groups (Friedmann et al., 2008); however, it is inadequate for the current research question for the following reasons. First, it is a comparison of groups and thus requires the independent variable of interest to be categorical. While effect sizes of different assessment waves may be of tangential interest to the research topic, the primary question relates to the continuous variables of smoking quantity and nicotine dependence. Second, standardized mean differences cannot determine local effect sizes, that is, individual effect sizes of particular variables within a multivariate model that includes other categorical and continuous independent variables.
