It is customarily employed to analyze large scale national surveys, including the National Health Interview Survey, the National Health and Nutrition Examination Survey, Drug Abuse Resistance Education survey, and the Behavior Risk Factor Survey. Although simple random samples performed with replacement use a normal distribution, random samples without replacement are based on a hypergeometric distribution.
Stratification is utilized to focus the survey on the target population.
The population may be stratified by income, if income is deemed crucial in the analysis. A cumulative frequency distribution of median household income may be computed and the distribution may be divided into low, medium, and high segments. Random samples may be conducted within each of these three strata to assure a proper representation of the population income distribution. When large surveys are conducted, there is often clustering of the population into areas. Clusters are selected by random sampling, then random samples are taken within the selected clusters.