This apparent inconsistency regarding whether pre-intervention alcohol use levels play a role in PFI efficacy may be attributed to several methodological issues. First, some of the previous studies may have lacked the necessary power to detect moderation effects due to insufficient overall sample size. Whereas many clinical trial studies are designed to have enough power to detect treatment (i.e., main) effects, few have enough power to detect differential efficacy across subgroups (i.e., moderation effects) (see Pocock, Assmann, Enos, & Kasten, 2002 for a review). The power to detect moderation is also affected by subgroup sample sizes, restriction in predictor variable range, magnitude of the moderating effect (Aguinis & Stone-Romero, 1997), and measurement error (Sackett, Harris, & Orr, 1986). Therefore, some existing studies may have inadvertently restricted the range of observations by screening out those whose baseline drinking levels were at lower ends of the spectrum, which may have resulted in lowered power. Second, previously reported findings are often based on univariate/bivariate analysis although pre-intervention drinking levels are generally known to be confounded with other individual and situational factors (e.g., gender).
