Chapter Seven - Miami Dade College

If the expected proportion is more than half, then plan the sample size based on the proportion expected NOT to have the characteristic. That is, switch the roles of P and 1-P. If P or 1-P is very close to 0 or 1 (i.e. the characteristic of interest is rare or happens most of the time), the sample size formula of N = 1 / E2 is not appropriate. ................
................