Aims: To assess external validity characteristics of studies from two Cochrane systematic reviews of the effectiveness of universal family-based prevention of alcohol misuse in young people.

Methods: Two reviewers used an a priori developed external validity rating form and independently assessed three external validity dimensions of generalizability, applicability, and predictability (GAP) in randomized controlled trials.

Results: The majority (69%) of the included 29 studies were rated ‘unclear’ on the reporting of sufficient information for judging generalizability from sample to study population. Ten studies (35%) were rated “unclear” on the reporting of sufficient information for judging applicability to other populations and settings. No study provided an assessment of the validity of the trial endpoint measures for subsequent mortality, morbidity, quality of life or other economic or social outcomes. Similarly, no study reported on the validity of surrogate measures using established criteria for assessing surrogate endpoints.

Conclusions: Studies evaluating the benefits of family-based prevention of alcohol misuse in young people are generally inadequate at reporting information relevant to generalisability of the findings or implications for health or social outcomes. Researchers, study authors, peer reviewers, journal editors and scientific societies should take steps to improve the reporting of external validity in prevention trials.