The matching number can be misleading
A health record may contain a mailing ZIP Code, while a population table reports a ZCTA. Joining them by the same five digits can look straightforward without ensuring that the people and boundaries align.
Some ZIP Codes represent delivery arrangements that do not correspond neatly to a residential area.
Check the geography before calculating a rate
- Identify whether each source uses ZIP Codes or ZCTAs.
- Check the year and boundary version.
- Review any crosswalk and its allocation assumptions.
- Examine unmatched or unusual codes.
A crosswalk can help, but it does not make every individual address assignment exact.
Do not let the map overstate precision
If records are assigned approximately, the display should not imply exact neighborhood boundaries or household locations. Small-area rates also need suitable denominators and attention to unstable counts.
For a local planning question, census tracts or another consistent geography may be more appropriate when available. The choice depends on the data and intended decision.
The Census Bureau's ZCTA documentation explains the construction and limits. Reading that definition before merging tables can prevent a polished map from combining a postal code numerator with a statistical-area denominator that does not describe the same population.
