Sample size
Sample size is calculated for the two levels of sampling, first the number of villages to be sampled and then the number of ponds to be sampled. The number of villages to be sampled depends on the sensitivity and the specificity of the test used to classify villages as infected or not infected. As the ‘test’ used in each village is really just another survey, the sensitivity is equal to the confidence and the specificity is equal to the power of the village-level survey. It is possible to adjust both confidence and power by changing the sample size in the village survey (number of ponds examined), which means that we can determine, within certain limits, what sensitivity and specificity we achieve.
This allows a flexible approach to sample size calculation.