“How big a sample do I require?” is one of the most frequently asked questions by investigators. Sample size calculation for a study estimating a population prevalence has been shown in many books (Daniel, 1999, Lwanga and Lemeshow,
1991). The aim of the calculation is to determine an adequate sample size to estimate the population prevalence with a good precision. It can be calculated using a simple formula as the calculation needs only a few simple steps. However, the decision to select the appropriate values of parameters required in the formula is not simple in some situations. In this paper, we highlight the problems commonly encountered, and give recommendations to handle these problems.