Assuming a 2% incidence of pelvic inflammatory disease in the control group, we needed a sample of 4122 women to detect a relative risk of 0.48 with 80% power and 5% significance. We had difficulty with recruitment,14 (link)
15 (link) however, as we were asking women who were not attending college for health reasons to provide vaginal samples that might not be tested for a year. Two studies9 (link)
19 (link) suggested a higher rate of pelvic inflammatory disease, enabling us to revise down our sample size calculations. Assuming a 3% incidence of pelvic inflammatory disease in the control group,9 (link)
19 (link) we needed a sample of 2274 women to detect a relative risk of 0.444 (link)
13 (link) with 80% power and 5% significance. Recruiting 2500 women allowed for 10% loss to follow-up.
For the primary analysis we estimated the relative risk of developing pelvic inflammatory disease in the 12 months after recruitment to the screened group compared with the deferred screening control group. In secondary analyses we examined the proportion of control women with untreated chlamydial infection who developed pelvic inflammatory disease within 12 months. We used exact methods to compute confidence intervals for unadjusted relative risks.20 (link) To adjust the relative risk of pelvic inflammatory disease for symptoms at baseline we also carried out an exploratory binomial regression using Stata version 10. Thirty five samples randomly allocated to the screening group were unintentionally put in the freezer and not tested for C trachomatis for 12 months. All participants were, however, analysed according to their original group allocation.
15 (link) however, as we were asking women who were not attending college for health reasons to provide vaginal samples that might not be tested for a year. Two studies9 (link)
19 (link) suggested a higher rate of pelvic inflammatory disease, enabling us to revise down our sample size calculations. Assuming a 3% incidence of pelvic inflammatory disease in the control group,9 (link)
19 (link) we needed a sample of 2274 women to detect a relative risk of 0.444 (link)
13 (link) with 80% power and 5% significance. Recruiting 2500 women allowed for 10% loss to follow-up.
For the primary analysis we estimated the relative risk of developing pelvic inflammatory disease in the 12 months after recruitment to the screened group compared with the deferred screening control group. In secondary analyses we examined the proportion of control women with untreated chlamydial infection who developed pelvic inflammatory disease within 12 months. We used exact methods to compute confidence intervals for unadjusted relative risks.20 (link) To adjust the relative risk of pelvic inflammatory disease for symptoms at baseline we also carried out an exploratory binomial regression using Stata version 10. Thirty five samples randomly allocated to the screening group were unintentionally put in the freezer and not tested for C trachomatis for 12 months. All participants were, however, analysed according to their original group allocation.
