Abstract
BACKGROUND: Most health promotion trials in cancer screening offer limited evidence of external validity. We assessed internal and external validity in a nationwide, population-based trial of an intervention to promote regular mammography screening. METHODS: Beginning in September 2000, study candidates age 52 years and older (n = 23,000) were randomly selected from the National Registry of Women Veterans and sent an eligibility survey. Consistent with intention-to-treat principles for effectiveness trials, we randomly assigned eligible respondents and nonrespondents to one of five groups. We mailed baseline surveys to groups 1-3 followed by intervention materials of varying personalization to groups 1 and 2. We delayed mailing baseline surveys to two additional control groups to coincide with the mailing of postintervention follow-up surveys to groups 1-3 at year 1 (group 4) and year 2 (group 5). Mammography rates were determined from self-report and Veterans Health Administration records. To assess internal validity, we compared groups on participation and factors associated with mammography screening at each stage. To assess external validity, we compared groups 3, 4, and 5 on mammography rates at the most recent follow-up to detect any cueing effects of prior surveys and at the respective baselines to uncover any secular trends. We also compared nonparticipants with participants on factors associated with mammography screening at the trial's end. RESULTS: We established study eligibility for 21,340 (92.8%) of the study candidates. Groups 1-3 were similar throughout the trial in participation and correlates of mammography screening. No statistically significant survey cueing effects or differences between nonparticipants and participants across groups were observed. Mammography screening rates over the 30 months preceding the respective baselines were lower in group 5 (82.3% by self-report) than in groups 1-4 (85.1%, P = .024, group 5 vs groups 1-4 combined), suggesting a decline over time similar to that reported for US women in general. CONCLUSION: This systematic assessment provides evidence of the trial's internal and external validity and illustrates an approach to evaluating validity that is readily adaptable to future trials of behavioral interventions.