# Table 1 Parameter values describing the natural history of chlamydia infection, PID development and the screening intervention

Parameters

Baseline values

Explanation

Sensitivity analysis

Source

Distribution

Parameters

Model parameters

λ

Force of infection (per day), calculated using*

1/r

365

Mean duration of infection (days) [6, 16]

N(μ,σ2)

μ=365

σ2=752

Consensus

p

5.7%

Prevalence at baseline 

Bin(n,p)

n=2519

$\text{p}=\frac{143}{2519}$



α

Effective testing rate (per day), calculated using

c

22.2%

Coverage of testing uptake (per year) 

Bin(n,p)

n=2377

$\text{p}=\frac{527}{2377}$



δ

8.0%

Treatment failure 

U(a,b)

a=0%

b=50%

Consensus

f

estimated

Fraction of women becoming infected with chlamydia who will develop PID

Input parameter

x

30.0%

Proportion of PID cases due to chlamydia in control group 

Bin(n,p)

n=23

$\text{p}=\frac{7}{23}$



1. *In the absence of the tria (α=0), to observe chlamydia prevalence p at steady state: $\lambda =\frac{p}{1-p}r$.
2. Reported uptake of chlamydia testing c during the follow-up period (outside of the trial) is reduced by the proportion with treatment failure δ, which results in the effective testing rate $\alpha =\frac{-ln\left(1-\left(1-\delta \right)c\right)}{365}$ per day .
3. N(μ,σ2), normal distribution (mean, variance); Bin(n,p), binomial distribution (size, probability); U(a,b), uniform distribution (minimum, maximum). 