Reply #63: This is the tricky part

[Febble]

There is no reason to suppose that simple non-random sampling (i.e. departure from strict Nth voter protocol) would induce bias in a particular direction unless there was also an underlying tendency for willing-looking voters also to tend to be Kerry voters. Given the latter, then non-random selection might be expected to exacerbate the underlying bias.

So the hypothesis is something like: if Kerry voters were more likely to look like willing participants, then the more leeway the interviewer had for selecting willing-looking (as opposed to Nth) voters, the more pro-Kerry would be the poll (and the redder the shift in the vote). The E-M report tells us that WPE was more negative where various factors likely to introduce leeway were present. In this sense, the participation bias theory is supported (I'm not terribly happy with the term non-response bias - it is possible that there was non-response bias as well, in the strict sense of more Bush voters actually overtly refusing to participate than Kerry voter, but the factors noted in the E-M report suggest selection bias rather than non-response bias in the usual sense of the term - though there is a sense in which looking unwilling to participate can be thought of as a form of non-response bias).

So yes, your first statement looks right to me, except that I will be even more specific and say that the discrepancy in the direction of greater proportion of Kerry responses than Kerry votes was greater where yadda yadda....