Participation, retention, and associated factors of women in a prospective multicenter study on Chlamydia trachomatis infections (FemCure).

Prospective studies are key study designs when attempting to unravel health mechanisms that are widely applicable. Understanding the internal validity of a prospective study is essential to judge a study's quality. Moreover, insights in possible sampling bias and the external validity of a prospective study are useful to judge the applicability of a study's findings.