Our findings seem to have a number of implications for CC research. First, comparisons between different instructional approaches that do not explicitly take selection bias into account should be considered inappropriate. We have demonstrated here that selection bias has nontrivial effects on their outcomes. Second, we suggest that conclusions reached in the reviews of CC studies published in the past 20 years should be used cautiously.7–11,15,16 We have shown here that robust effects of PBL were actually masked in many of the studies included in these reviews. The possibility that other CC studies have suffered from the same shortcomings cannot be excluded, particularly because problem-based schools other than Maastricht have also begun to report lower attrition rates than conventional comparison schools.30–32 It is, therefore, unfortunate that calls to discard PBL, based on these reviews, have been relatively influential.11–13 We believe we have shown that these calls lacked solid empirical foundation. Finally, our findings demonstrate that PBL, as a serious approach to improving medical education, may deserve another, less biased look.