Table 1.

Population density (people/km²) and urbanization (percent of urban area) of all zip code areas in Illinois and of the 214 zip code areas included in the analysis. The p value was calculated from the student t test.

	Illinois	This study	P value
Number of zip code areas	1383	214	–
Average Population Density (People per km2)	543.16	2340.18	<0.0001
Median % Urban Area* in zip code areas	0	100.00	<0.0001

“Urban Area” is defined by the Census Bureau [17]. Cities with a population of >50,000 residents and adjacent satellite cities/towns with >2500 residents were categorized as urban area.