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".


This calculator provides the user with the number of animals to test, and the number of positive test animals at which to call a herd positive, in order to achieve a given herd level sensitivity and specificity. It requires knowledge of the individual animal test sensitivity and specificity, herd size and the minimum within herd (or animal level) prevalence for a truly positive herd.

Warning: Where achieving desired herd sensitivity and specificity is difficult (or not possible) the calculation may take some time.


Please ensure the sum of test sensitivity and specificity is at least than 100%

Selecting a row in the table will highlight the corresponding bars in the plot.

This figure presents, graphically, the same results as are provided in the table. Results are presented for each combination of Desired Herd Specificity, and Desired Herd Sensitivity.

These results are the number of animals (height of full bar) required to be tested and the cutpoint (height of solid bar) for the number of positives above which the herd should be deemed test positive.

The achieved herd sensitivity and specificity are the lowest levels that can be achieved that are higher than (or equal to) the desired herd sensitivity. This is because we can only work in whole numbers of animals/individuals. Therefore the achieved herd sensitivity and specificity are higher than desired

The 'cutpoint' describes the number of test positive animals above which the user should categorise the herd as test positive. If the number of test positive animals equals (or is less than) the cutpoint then the herd should be deemed test negative.