Implementing the New Sample Design for the Current Employment Statistics Survey

Business Economics, Oct, 2000 by Patricia M. Getz

Methodological Issues and How They Are Being Addressed.

The new CES sample design

The new design is a stratified, simple random sample of worksites, clustered by UI account number. The UI account number is a major identifier on the BLS longitudinal database of employer records. This database serves as both the sample frame and the benchmark source for the CES employment estimates. The sample strata, or sub-populations, are defined by state, industry and employment size and yield a state-based design. The sampling rates for each stratum are determined through a method known as optimum allocation, which distributes a fixed number of sample units across a set of strata to minimize the overall variance, or sampling error, on the primary estimate of interest. The statewide total non-farm employment level is the primary estimate of interest, and the new design gives top priority to measuring it as precisely as possible. In other words, it minimizes the statistical error around the statewide total nonfarm employment estimates.

For the CES redesign, the number of sample units drawn for each state was fixed to the approximate size of the existing CES sample, the sample size supportable by current program resources. This sample size supports the publication of considerable industry and geographic detail within a state and provides for highly reliable national CES estimates at the total nonfarm and detailed industry levels.

The sampling frame and the CES sample are updated twice a year with new quarters of UI-based universe data. This helps to keep the sample upto-date by adding business births and deleting business deaths. In addition, the new design specifies an annual update process that includes sample frame maintenance and the redrawing of the entire sample for the first quarter of each year. Frame maintenance provides for the updating of industry, size class, and metropolitan area designations and for the merging of semi-annual birth samples into the overall frame. A high degree of overlap is expected at each annual update because all UI accounts are ordered on the frame with permanent random numbers (PRN). This technique assigns random numbers to all UI accounts on the universe frame at the time they first appear and then orders the frame by PRN. The allocation for each sampling cell is fulfilled by working down the ordered PRN list until the full complement of needed units is drawn. Because the random numbers are perman ent and thus remain in essentially the same order on the frame, this technique minimizes cancellation of existing sample units and the need to solicit replacement units.

Modifications to the research design

The new sample design, initially developed in a two-year research phase, was first tested under near-ideal conditions, using historical UI universe microdata files as a proxy for monthly collection of CES sample data from respondents. Thus, the research phase allowed BLS to define the best design and estimators from a purely methodological viewpoint.