Objective: An experimental trial was used to assess the effectiveness of a self-help book and a personalized assessment-feedback intervention, both separately and in combination with each other, in a general population survey.
Method: Participants (N = 86; 66.3% male) were recruited through a random digit dialing telephone survey conducted by the Centre for Addiction and Mental Health, Toronto, Ontario, Canada. Respondents were randomly assigned to one of four conditions in a two-by-two factorial design: "no-intervention" control group, "personalized feedback only," "self-help book only" and "both personalized feedback and self-help book." Respondents were followed up in 6 months' time, and differences in drinking status were compared between experimental conditions using a multivariate analysis of covariance with baseline drinking severity as the covariate.
Results: Support was provided for an interaction hypothesis in which respondents who received both interventions reported significantly improved drinking outcomes at 6-month follow-up, compared with respondents who received just one of the interventions or who received no intervention.
Conclusions: Because respondents were recruited from a representative sample of the general population into a randomized trial with a no-intervention control group, this research design maximized both external and internal validity in examining the effectiveness of self-help interventions.