ZIP Code and ZIP Code Tabulation Area Linkage: Implications for Bias in Epidemiologic Research

To our knowledge, no agreed-upon best practices exist for joining U.S. Census ZIP Code Tabulation Areas (ZCTAs) and U.S. Postal Service ZIP Codes (ZIPs). One-to-one linkage using 5-digit ZCTA identifiers excludes ZIPs without direct matches. "Crosswalk" linkage may match a ZCTA to multiple ZIPs, avoiding losses.

We compared noncrosswalk and crosswalk linkages nationally and for mortality and health insurance in California. To elucidate selection implications, generalized additive models related sociodemographics to whether ZCTAs contained nonmatching ZIPs.

Nationwide, 15% of ZCTAs had nonmatching ZIPs, i.e., ZIPs dropped under noncrosswalk linkage. ZCTAs with nonmatching ZIPs were positively associated with metropolitan core location, lower socioeconomics, and non-White population. In California, 34% of ZIPs in the mortality and 25% in the health insurance data had ZCTAs with nonmatching ZIPs; however, these ZIPs constitute only 0.03% of total mortality and 0.44% of total insurance enrollees.

Our study findings support the use of crosswalk linkages and ZCTAs as a unit of analysis. One-to-one linkage may cause bias by differentially excluding ZIPs with more disadvantaged populations, although affected population sizes seem small.