Statistical errors invalidate conclusions in "Caffeine and unsaturated fat diet significantly promotes DMBA-induced breast cancer in rats".

The conclusion drawn in a recent paper by Minton and associates (Cancer 1983; 51:1249-1253), that caffeine and an unsaturated fat diet significantly promoted dimethylbenz(a)anthracene (DMBA)-induced breast cancer in rats, is based on fallacious statistical reasoning. Minton and associates based their conclusion on the mean latency to first tumor appearance for rats diagnosed with tumors. However, evaluation of percentages of tumor-bearing rats yields contrary results. Using data from a totally negative hypothetical two-group experiment, we demonstrate how such "conflicting" results could arise from differential intercurrent mortality in the two groups. The correct statistical analysis of this hypothetical experiment allows for differential intercurrent mortality and no conflict arises; the data of Minton and associates need to be analyzed by these methods.