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Four-Digits or No-Digit Social Security Numbers -- Impact on the National Agricultural Statistics Service Record Linkage Maintenance Processes

The National Agricultural Statistics Service (NASS) relies on Social Security Numbers (SSNs) and Employer Identification Numbers (EINs) as important matching variables for use in record linkage processing and other list maintenance activities that are conducted in order to maintain a high quality list of U.S. farmers and ranchers. Record linkage is used to match the NASS list frame to new lists and administrative data sources in order to identify new farmers and ranchers not previously identified by NASS. Record linkage is also used to match the list frame to administrative sources for maintenance purposes, for example updating telephone numbers on the list frame. However, maintaining nine digit SSNs/EINs on the list frame is a PII security concern for NASS. This research evaluated the potential impact of eliminating SSNs/EINs from the list frame or of only maintaining four digit SSNs/EINs on the various list building and maintenance record linkage processes. Overall, the results showed that using four digit SSNs/EINs for record linkage would result in missing 1 to 2 percent of the actual SSN matches and 4 to 8 percent of the actual EIN matches. However, having no SSNs/EINs for record linkage would result in missing 4 to 6 percent of the actual SSN matches and 9 to 13 percent of the EIN matches. In general, the percentage of missed matches will increase as the size of the data sets being matched increases. Since NASS processes several record linkage projects per state per year, the cumulative effect of either approach on the quality of the list frame is a concern.