If you enter a precision of 5%, Sampsize will return the
sample size needed for 95% (default) or any other confidence interval where
the upper limit equals prevalence + precision and the lower limit
equals prevalence  precision.
The population size is the total size of the population from which
a sample will be drawn for the survey. If the population size is
small, the correction for finite population will result in a reduced
sample size.
Entering 50 for the estimated prevalence will result in
the highest sample size. Entering 0 for the population
size will result in Sampsize using an infinite
(very large) population size. If the population is
infinite, sampsize will also return the exact binomial
confidence limits given the returned sample size and
entered prevalence. These limits should be close to the
specified limits. If the exact bounds are wider than the
specified ones, then the formula for estimating the sample
size is likely inappropriate, and an alternative course of
action is to test different combinations of sample size
and number of observed events (such as the number of
patients with the studied disease) and calculating an
exact confidence interval given the
combination.
