The analysis of data from samples constitutes a major proportion of contemporary research in the social sciences. For example, researchers use sample data from the U.S. population to estimate, with specified levels of confidence and precision, quantities such as average household size, the proportion of Americans who are unemployed during a given month, and the correlation between educational attainment and annual earnings among members of the labor force. Sample-based estimates are called sample statistics, while the corresponding population values are called population parameters. The most common reason for sampling is to obtain information about population parameters more cheaply and quickly than would be possible by using a complete census of a population. Sampling also is used sometimes when it is not feasible to carry out a complete census. For example, except perhaps in a few nations with population registers, attempts to carry out a census...