From Hauser, Robert M. 2002. "Meritocracy, cognitive ability, and the sources of occupational success." CDE Working Paper 98-07 (rev). Center for Demography and Ecology, The University of Wisconsin-Madison, Madison, Wisconsin.
http://www.iqcomparisonsite.com/Occupations.aspx | Research Article [PDF] - via MoPo
1) Sample size: data are collected for sample sizes of "30 cases or more" for each occupation; hmmm, that really seems like a broad and effective sampling to me...not.
2) Sample population: "Based on [male/female] Wisconsin [University] graduates and siblings who reported a job during 1992-94". Yeah, we all know that Wisconsin is a hub of science, engineering and technology.... right.... maybe that's why MDs are the smartest ones there... because it's Wisconsin for godssake! I'm pretty sure the engineers who designed the Mars rover have an IQ higher than 130.