Translations:Advanced Field Epi:Manual 2 - Diagnostic Tests/193/en
Where NPV is the negative predictive value of the test in the population of origin, Se and Sp are animal-level sensitivity and specificity respectively, Prev is true disease prevalence and m is the number of animals tested. As sample size increases the probability that the group will all test negative decreases, so that the overall risk associated with a group can be reduced by increasing the sample size. However, if all animals do test negatively the probability that one or more are actually infected increases (assuming that they are from an infected population), as shown in Figure 4.6.