We want to conduct a national study to estimate a prevalence of certain disease. we still confused about the sample size calculation and the criteria of selecting a representative sample. First: is the formula of sample size differ from the formula of representative sample calculation? Second: The sample will be among a selected age group from different regions of the country, how can we ensure that the sample is representative for the whole country? Third: if we want to select the sample from schools (private and public) do we call it a cluster group?