Table 1.

Parameter values describing sample size calculation, the natural history of chlamydia infection and PID development

Parameters		Re-examine POPI trial		Generic sample size calculation		Source
		Baseline values	Range	Baseline values	Range
Sample size calculation
inc	PID incidence (per year)	1) 2 % & 2) 3 %a		calculated†		POPI trial [14]
RR	Relative risk	1) 0.48 & 2) 0.44a		calculateda		POPI trial [14]
t	Follow-up time (in days)	365		365	90–540‡	POPI trial [14]
α	Significance level	5 %		5 %		POPI trial [14]
1-β	Power	80 %	10–90 %†	80 %		POPI trial [14]
Infection parameter
λ	Force of infection (per day)	calculated§		calculated§
1/r	Duration of infection (in days)	365	365 ± 75	365	365 ± 75	Model [17]¶
p	Prevalence of infection	7 %	3–10 %‡	7 %	3–10 %‡	POPI trial [14]
Progression to PID
f	Fraction of women with chlamydia who progress to PID in absence of testing	calculated†		10 %	7–13 %	Model [16]¶
1/γ	Infection progression (in days)	calculated#		calculated#

^aFor the three types of progression, PID incidence is used for the control group and RR is calculated per type

^†The fraction f and the PID incidence inc in the control group satisfy the equation: fprt = inc

^‡Range determined by agreement among authors

^§In the absence of the trial, to observe chlamydia prevalence p at steady state: $\lambda =\frac{pr}{1-p}$

^¶Results of a mathematical model which used published trial data

^#To achieve the same cumulative PID incidence in all three processes in absence of the trial: $\gamma =\frac{fr}{1-f}$

PID, pelvic inflammatory disease; RR, relative risk