For a nice article on ACS sampling, please see: http://www.esri.com/news/arcuser/0408/samplesize.html

Sample Size Matters:  Caveats for users of ACS tabulations  by Lynn Wombold, ESRI Chief Demographer

ArcUser Winter 2008   pps 9-11

Elaine Murakami

FHWA Office of Planning

206-220-4460