As lymonl puts it:
But aren't the exit polls all done by human beings, not machines? How will you know how African-Americans really voted if they tell you on the way out that they voted for Clinton? You wouldn't be able to attribute the entire disparity (heavier vote total for Obama than the exit polls indicated) because whites may have lied to exit pollsters too.
Mark writes in response:
The problem with both arguments is that voters don't "tell" exit pollsters anything. Interviewers hand respondents a paper form, which they fill out privately and drop into a "ballot box."
On the other hand, the characteristics of exit poll interviewers (race, gender and age) may have some influence on whether voters agree to participate in the survey. Historically, exit pollsters depend on mostly younger interviewers and have, as a results, had the hardest time gaining cooperation from older voters. While exit pollsters attempt to correct for such "non-response bias" by weighting, some distortions may remain.
So on the question of racial polarization in today's vote, it would be helpful to attempt to verify the exit poll findings with actual results in heavily African-American precincts.
Something to keep in mind if you see exits this evening that leave you either giddy or demoralized...