Misalignment Between ZIP Codes and Municipal Boundaries: A Problem for Public Health

Richard C. Sadler
Assistant Professor, Division of Public Health, Michigan State University

While useful for mail delivery, ZIP Codes are flawed as a geographic metric for public health research. This paper quantifies the magnitude of potential error inherent in using ZIP Codes as a unit of analysis in the state of Michigan. ZIP Codes are intersected with municipality boundaries in ArcGIS to determine the degree of misclassification. Results showed that 49 percent of the population had their municipality misclassified by their ZIP Code. This creates potentially huge errors when ZIP Code is the only geographic identifier, because actual exposure may vary from the exposure to which an individual is assigned based on ZIP Code. The Flint, Michigan, Water Crisis is a prime example of this error and the need to consider finer units of analysis whenever possible. Collaboration with experts in geographic information science is therefore essential for any public health research project where location is a factor.