Specifically at this link:

http://www.bls.gov/opub/hom/homch1_f.htm

The short answer is, yes, they extrapolate from a sample. 72,000 households from 754 sample areas. Crude calculations here, but some quick linear analysis says they found 1 janitor with a PhD, JD, MBA, or another professional degree.

Nice perspective, I can easily imagine finding one janitor with PhD in 200K town.

There is absolutely nothing surprising about this (in contrast to seemingly high 5000 PhD janitors in the whole US).