Is there any specific reason to believe that participation bias does explain the discrepancies?
Yes, beyond the facts that participation bias is common, that past exit polls have overstated Democratic performance, and that the exit poll discrepancies don’t correlate with pre-election poll discrepancies, “swing” from 2000, or electronic voting machine use, there is also some evidence indicating participation bias in 2004. Specifically, some patterns in the exit poll discrepancies fit well with participation bias. For instance, “red shift” is much greater in precincts where interviewers stood further from the polling place. The average WPE increases from -5.3 where interviewers stood inside the polling place to -12.3 where interviewers stood more than 100 feet away (evaluation report, p. 37). This result is consistent with the conjecture that Kerry voters were more eager to participate in the exit polls than Bush voters were — and this disparity was more manifest in polling places where it was easier to avoid the interviewers entirely. The recorded completion rate falls from 59% w
