How many samples should I take?
This spreadsheet provides a guide to the number of samples that should be taken to find one or more positive samples.
The difficult issue is determination of the likely number of infected animals in a population
This presumes that the test has a 100% sensitivity
The formula is n=(1-(1-p(1/D) ))*(N-0.5(D-1)
Where n = required number of samples
D= number of infected animals
N= total number of animals (groups, house, farm
p= degree of certainty required - for example 0.95 for 95%.
See sensitivity and specificity