Sample size for demonstration of freedom (detection of disease) in large populations

Test sensitivity

Desired herd sensitivity

0.9 0.95 0.98 0.99

Design prevalence

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This utility calculates the sample size required to provide a desired probability of detecting disease (herd-sensitivity) at the specified design prevalence, for a large (infinite) population, assuming a test of known sensitivity and 100% specificity. These analyses use the method from Martin et el. (1992) (Prev Vet Med, 14:33-43), assuming known test sensitivity, perfect test specificity and a cut-point number of reactors of 1 to declare a population infected (i.e. to be recognised as free there must be zero positive reactors). See also Richards, MS (1983). Proceedings of the 3rd International Symposium on Veterinary Epidemiology and Economics, pp 567-570.

Inputs are:

• Test sensitivity;
• Desired herd-sensitivity; and
• Design (target) prevalence.

Outputs are:

• The number of samples required to be tested to provide the desired herd-sensitivity for the given design prevalence; and
• A table and graph of sample sizes required to provide the desired herd-sensitivity, for the specified test sensitivity and a range of population sizes and design prevalences.