James P. Scanlan is an attorney specializing in the use of statistics in litigation. He has published about 60 articles on legal or public policy issues. About half have pertained to the use of statistics in the law and the social and medical sciences, especially regarding the patterns by which standard measures of differences between outcome rates tend to be systematically affected by the prevalence of an outcome. Most notably, the rarer an outcome the greater tends to be the relative difference in experiencing and the smaller tends to be the relative difference in avoiding it, a pattern termed “Scanlan’s Rule” by scholars in the UK. Thus, for example, improvements in health or healthcare tend to decrease relative differences in favorable health outcomes, while increasing relative differences in the corresponding adverse outcomes; increasing loan approval rates tends to decrease relative differences in approval rates while increasing relative differences in rejection rates. Without recognizing this and related patterns it is not possible to soundly interpret data on group differences in outcome rates.