This app contains different calculators to find the sample size needed to:
A within herd sample size calculator for designating a herd as positive or negative based on a number of animals tested using an imperfect test. Also known as "demonstrating freedom of disease".
A two stage sample size calculator, which is designed to help design surveys to estimate Herd Level Prevalence when the individual test is imperfect.
A predictive values calculator, which will calculate the positive and negative predictive values, from a given sensitivity and specificity, for a range of prevalence estimates.
A Herd sensitivity/specificity calculator for calculating the herd sensitivity and specificity based on the number of animals sampled and the number of positive animals at which the herd will be considered positive. As with the above calculators this is designed for an imperfect test.
The number of test positives divided by the number of animals tested gives the 'apparent prevalence'. If we know the test sensitivity and specificity then we can calculate an estimate for the 'true prevalence' which is what this calculator does.