Another difference in the studies involved the use of estrogen hormone replacement therapy among a significant fraction of the participants in the European studies. The number of participants in prevention trials is a major determinant of the likelihood of reliably detecting any differences that may exist between the tamoxifen and placebo groups. The larger the groups involved, the more likely it is that true differences between the groups will be detected with high probability. In contrast, the smaller the number of participants the more likely it is that any differences seen may be due to chance alone, or that real differences might be missed. The biggest impact of a difference in the intrinsic risk of participants relates to the number of cases that are observed in the study.
More cases occur in a study involving women at increased risk.
This difference in case numbers between the groups is very significant. For the larger of the European studies, the numbers of cases were 36 and 34, respectively. For the smaller of the two studies, the numbers of cases were 22 in the placebo group and 19 in the tamoxifen group. The larger number of cases in the BCPT provides more confidence that the observed differences are real. Tamoxifen can interact with the same receptor on cells that binds estrogen.