A Public Use Microdata Area (PUMA) is a geographic unit used by the US Census for providing statistical and demographic information.
For example, the ACS publishes detailed data every year, but due to their sampling procedure only publishes data for census area that have more than 65,000 People.
Only seven of the 55 counties of West Virginia were large enough to receive estimates from the 2006 ACS.
In contrast, all 12 PUMAs that partition West Virginia received 2006 ACS estimates.
The state governments drew PUMA boundaries for the 2000 Census, to allow reporting of detailed data for all areas.