Unit is the Census ZCTA; PO-box-only ZIPs appear as gaps. ACS and IRS are ZCTA-native. Homicide rate is county-level (every ZCTA in a county shares the value). Test scores are school district-level (every ZCTA in a district shares the value; district picked by largest-area overlap). Life expectancy is tract-level, aggregated to each ZCTA by area-weighted mean.
Sources: Census ACS B19013, IRS SOI, County Health Rankings 2025 (homicides via CDC WONDER), SEDA v6.0 (Stanford test scores), CDC USALEEP (life expectancy, 2010-2015), TIGER ZCTA520