Empirical comparison of the results of randomized controlled trials and case-control studies in evaluating the effectiveness of screening mammography.

The gold standard for evaluating screening programs is the randomized controlled trial (RCT). Case-control studies are easier to perform but their role in this area is controversial. The purpose of this article is to compare empirically the results of RCTs and case-control studies conducted to evaluate the efficacy and effectiveness of screening mammography and examine possible explanations for differences in their results. We located eight RCTs and five case-control studies of screening mammography. For women aged 40-74 years at screening, comparison of the summary risk estimates of the RCTs (0.76, 95% CI: 0.69-0.83) with that of the case-control studies (0.44, 95%, CI: 0.38-0.50) showed RCTs to have a significantly higher summary risk estimate than case-control studies (ratio = 1.74, 95% CI: 1.48-2.04). It is notable that the RCTs were compromised in most instances by low compliance rates (50-80%) in the treatment groups and by significant use of screening in the control groups (20-30%). Adjustment of the RCT results for these cross-overs yields results that are in reasonable agreement with the summary estimate for the case-control studies. These findings support the use of case-control studies to estimate the efficacy of mammographic screening where RCTs are not feasible. They suggest that the efficacy of mammography in women aged 50 years and above is somewhat greater than the effectiveness measured by the intent-to-treat analysis of RCTs.