Re: Promoting Regular Mammography Screening I. A Systematic Assessment of Validity in a Randomized Trial

Extract

We congratulate del Junco et al. ( 1 ) 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 ( 1 ). We have three main comments about the study.

First, the assessed control group was also compared with two randomized groups of study participants for whom delayed baseline surveys were administered. Surely, this is a test of internal validity, rather than external validity, as was presented. This comparison was not directed toward any external population, but rather was undertaken to ascertain whether the assessed control group provides a valid comparison for the evaluation of the intervention conditions.