Rosenbaum focused on about 3,400 students who had not had sex or taken a virginity pledge in 1995. She compared 289 students who were 17 years old on average in 1996, when they took a virginity pledge, with 645 who did not take a pledge but were otherwise similar.

She based that judgment on about 100 variables, including their attitudes and their parents'' attitudes about sex and their perception of their friends'' attitudes about sex and birth control.