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Chapter 83: Sample Size for Proportions
Researchers are frequently concerned with determining sample size for problems involving estimating population proportions or percentages. When the sample size question involves the estimation of a proportion, the researcher requires some knowledge of the logic for determining a confidence interval around a sample proportion estimation (p) of the population proportion (π). For a confidence interval to be constructed around the sample proportion (p), an estimate of the standard error of the proportion (Sp) must be calculated and a confidence level specified.
The precision of the estimate is indicated by the value ZciSp. Thus, the plus or minus estimate of the population proportion is
If the researcher selects a 95% probability for the confidence interval, Zci will equal 1.96.