9 - Sample size calculation for fixed pool size and tests with known 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 known exactly (i.e. that there is no uncertainty about the values).

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%. For a pool size of five, 20 pools would be sufficient to provide the desired precision and confidence in the estimated prevalence, assuming asymptotic confidence limits were used. Suggested sample sizes are generally lower than those for a perfect test with corresponding pool sizes and the resulting asymptotic confidence limits van be substantially narrower than corresponding exact limits.