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

Table 2 Sample size and power calculations for different scenarios

	Change (%)	Sample size needed at baseline (80% power)		Expected power with sample size n = 2000 at baseline
	Change (%)	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%

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-)
Abbreviations: Ct+ chlamydia positive, Ct- chlamydia negative

ISSN: 1471-2334