We congratulate del Junco et al. for undertaking a dedicated study of validity in their mammography screening trial. We are particularly interested in their randomized studies of the possible effects of completing baseline surveys and in their conceptualization of validity. The study population was randomly sampled, and screening rates in the assessed control group were compared with secular trends among nonparticipants, providing a robust test of whether study participation altered screening behavior, and thus whether the findings have external validity.
Journal of the National Cancer Institute Vol. 101, Issue 14, p. 1029-1030