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Estimating prevalence

Sample size to estimate a true prevalence with an imperfect test

Calculate the sample size required to estimate true prevalence with a specified level of confidence and precision, assuming a test with imperfect sensitivity and/or specificity. The same method applies for estimating both animal and herd-level prevalence, with herd-sensitivity and herd-specificity substituted for animal-level values to estimate true herd-prevalence. The method is as described by:
Humphry RW, Cameron A, Gunn GJ, 2004. A practical approach to calculate sample size for herd prevalence surveys. Prev. Vet. Med. 65: 173-188.

Inputs are the assumed true prevalence, the desired level of confidence, the desired precision of the estimate and the assumed values for sensitivity and specificity of the testing regimen used. The desired precision of the estimate (also sometimes called the allowable or acceptable error in the estimate) is half the width of the desired confidence interval. For example if you would like the confidence interval width to be about 0.1 (10%) you would enter a precision of +/- 0.05 (5%).

To calculate sample size for herd-prevalence estimation, use herd-level values for assumed prevalence, sensitivity and specificity instead of animal-level values.

The program outputs the sample size required to estimate the true prevalence with the desired precision and confidence. Tables of sample sizes for a range of values for prevalence and precision and for sensitivity and specificity are also produced.

Note: Adjustment for finite population size has been removed from this page because using the adjustment may underestimate required sample size, unless this is also taken into account when estimating variance and resulting confidence interval. If you wish to continue using the adjustment, the original page is available here.