Skip to main content

Table 2 Sample size and power calculations for different scenarios

From: Study protocol of the iMPaCT project: a longitudinal cohort study assessing psychological determinants, sexual behaviour and chlamydia (re)infections in heterosexual STI clinic visitors

  Change (%) Sample size needed at baseline (80% power) Expected power with sample size n = 2000 at baseline
Ct+ Ct- Ct + (n = 300) Ct- (n = 1700)
Scenario 1 10% 470 611 60% > 99%
Scenario 2 15% 220 286 91% > 99%
Scenario 3 20% 129 167 > 99% > 99%
  1. Sample size and power calculations to detect a change in psychological determinants or sexual behaviour at six-month follow-up with at least 80% power, assuming 34% loss to follow-up in the chlamydia positives (Ct+) and 55% loss to follow-up in the chlamydia negatives (Ct-)
  2. Abbreviations: Ct+ chlamydia positive, Ct- chlamydia negative