"The National Institutes of Health convened a group of experts that year to address the question 'What is the scientific evidence on the effectiveness of latex male condom-use sic to prevent STD transmission during vaginal intercourse?'

Most of the relevant published studies, they found, didn't use the kind of experimental design you'd want to see – i.e., prospective design, in which the researcher compiles data by following the subjects' behavior going forward, rather than gathering data after the fact."