Estimate the true prevalence
of infection from survey testing results, using a test with imperfect
sensitivity and
specificity.
Sensitivity and specificity estimates are specified as Beta
probability distributions, with parameters alpha and beta. Beta probability distributions are
commonly used to express uncertainty about a proportion based on a random sample of individuals.
In this situation, if there are x "successes" out of n individuals examined, then the
alpha and beta parameters can be calculated as alpha = x + 1 and beta = n  x + 1. Alternatively,
alpha and beta can be calculated using the Beta distribution
utility, provided estimates of the mode and 5% or 95% confidence limits are available
from expert opinion or other data.
Simulated true prevalence estimates are calculated as described by:
Rogan and Gladen (1978). Estimating prevalence from the results of a screening test.
American Journal of Epidemiology 107:7176.
Outputs include a table and graphs summarising the probability distributions for true
and apparent prevalence and test sensitivity and specificity.
