P is the anticipated value of proportion. N is the size of the population. Summary Statements A sample size of 341 from a population of 3000 produces a twosided 95% confidence interval-with a precision (half-width) of 0.0400 when the actual proportion is near 0.2000.This report shows the calculated sample size for each of the scenarios.onfidence Intervals for One Proportion from a Finite Population. 116-4. © NCSS, LLC. All Rights Reserved. Example 1 – Finding Sample Size. Suppose a study is planned in which the researcher wishes to construct a two-sided 95% confidence interval for. the population proportion in a population of 3000 items.