EXAMPLE 7 The Current Population Survey

The population that the Current Population Survey (CPS) is interested in consists of all households in the United States (including Alaska and Hawaii). The sample is chosen in stages. The Census Bureau divides the nation into 2007 geographic areas called primary sampling units (PSUs). These are generally groups of neighboring counties. At the first stage, 754 PSUs are chosen. This isn’t an SRS. If all PSUs had the same chance to be chosen, the sample might miss Chicago and Los Angeles. So 428 highly populated PSUs are automatically in the sample. The other 1579 are divided into 326 groups, called strata, by combining PSUs that are similar in various ways. One PSU is chosen at random to represent each stratum.

Each of the 754 PSUs in the first-stage sample is divided into census blocks (smaller geographic areas). The blocks are also grouped into strata, based on such things as housing types and minority population. The households in each block are arranged in order of their location and divided into groups, called clusters, of about four households each. The final sample consists of samples of clusters (not of individual households) from each stratum of blocks. Interviewers go to all households in the chosen clusters. The samples of clusters within each stratum of blocks are also not SRSs. To be sure that the clusters spread out geographically, the sample starts at a random cluster and then takes, for example, every 10th cluster in the list.