10 - Sample size calculation for fixed pool size and tests with uncertain sensitivity and specificity

This option can be used to calculate the number of pools that need to be tested for various values for pool size, estimated prevalence and desired confidence and precision of the estimate. It assumes fixed pool sizes and that the true values of both sensitivity and specificity are not known exactly and have been estimated in a limited number of samples.

For this analysis, sample size was calculated to provide 95% confidence of estimating a true prevalence of 0.14 (14%) with a precision of 0.055 (5.5%). This is equivalent to the observed prevalence and precision when 162 samples from little red flying foxes in Queensland were tested individually, with 22 positive results (H. Field, pers com). Test sensitivity was assumed to be 90%, to demonstrate the effect of imperfect test sensitivity on sample size estimates and specificity was assumed to be 100%. To allow for uncertainty about the true values of test sensitivity and specificity, it was assumed that sample sizes of 50 and 10,000, respectively, were used to estimate these values. For a pool size of five, 59 pools would be sufficient to provide the desired precision and confidence in the estimated prevalence.