As many of you mappers know, when coloring filled maps using a measure you can get significantly different (and therefore misleading!) results depending on whether or not you control for the areas of the different shapes (states, counties, zips). For instance here is a filled zip code map of NY City and surrounds. This first one is color filled based on population within zip codes:
This next one is filled using population density (population/square miles):
I've attached a workbook and excel spreadsheet that includes population, population density, and square mile area of almost all of the US zip codes. (The area calculations were provided by a bit of wizardry from Richard.) We hope this serves as a good resource for some of you. Let me know if you do use it, and I'll work to keep it up to date.
One last note: This isn't just about population density. I'll be using it for things like [number of businesses]/[square miles] or [impressions]/[square miles]. Anything where density matters.