What is striking about these results is the rather consistent moderate effects reported on most outcomes. For every study, Table 1 systematically reports on alcohol outcomes such as drinking (use or frequency), binge drinking (use or frequency) and alcohol problems (presence or severity). The average effect size across all studies and all outcomes is d = 0.47. For a small number of studies, one or two of these outcomes are not reported. Assuming that they are not reported because they did not yield significant effects, we assign a d = 0 to missing effect sizes and still yield an average effect size of d = 0.43 across all studies and measures (0.26 for alcohol use; 0.35 for binge drinking (including two unreported studies) and 0.46 for problem drinking). Three studies report illicit drug use outcomes, and the average of these reported outcomes is a d = 0.37.
