Nov 06, 2020. How the polling may have failed in Pennsylvania.
This year, for the first time ever, a large number of Pennsylvanians
voted by mail, and Trump's campaign against voting by mail clearly
made an impact. Election returns show that, across Pennsylvania, Biden
took 75% of the vote-by-mail.
Most pollsters report their predictions according to those “likely to
vote.” From what I have read long ago, that made use of previous
voting – using either state records or self-report – and maybe
factoring in enthusiasm.
If you are a pollster, scrabbling to reach 1000 subjects when 9 out of
10 people won't talk to you on the phone, I think you quietly and
happily put the person who has voted by mail into the likely-to-vote
stack; this year, this state, that would have been a mistake that
created an bias of several points, towards Biden.
To see that, consider this example. By election week, “already voted”
could be half of your 1000, most of them who would have made it by any
standard. Now suppose that 100 of your “likely-to-vote” sample won
that qualification solely by telling you they have voted. If the rest
of the sample was 450-450, you have raised the totals to 525-475, an
unearned 50 person lead for Biden.
In retrospect, this is an obvious mistake. But I can see how it could
have crept in. The decision to pool Voted with Likely never made a
difference before, because: (a) "Voted" was a tiny number; (b) it was
always balanced evenly, more or less, between parties. Suddenly, in
2020, it is a big number and there is a big partisan split in mailing,
in states new to mail-in voting.
One reason that I suspect that this blunder has occurred is that I
have seen NO comments about what pollsters did with “already voted”
when they /could/ have informed us of what margins to expect in the
states in final contention. Steve Karnacky (MSNBC) is reporting on
what he has seen, when he says 75-25 statewide
As the final votes come in (Nevada, Pennsylvania, Arizona, Georgia),
the Finite Population Correction is difficult to apply because of the
mixing-in (it seems) of many so-called “provisional ballots” whose
propensities are unknown.