Eugene,  .......   Exit polls are a problem in two ways, only one of
which is "purely mathematical."

The math problem is the random variation that one gets in taking
samples. If one samples any population repeatedly, the several samples
will differ among themselves in ways that are rather predictable. The
standard Gaussian bell curve generates the theory of how these
variations are patterned. Of course, one must make some assumptions
about the nature of the population that is being sampled before any of
this logic can get going, and a second "of course" is that the
population of voters does not fit these assumptions perfectly. However,
the fit is close enough and this part of the problem is mathematically
well behaved. (Perhaps I should say that "it is close enough for
government work.")

The problem that is not purely mathematical (and this problem is the
serious problem) is that exits polls do not use random samples.
Depending on where one is in the country, people vary in their
willingness to respond to the pollsters. So those who do respond are
"self-selected" and thus a non-random sample; they are not typical of
the larger population of those who voted. At the worst, which does not
always happen, one gets the equivalent of a "Literary Digest" poll.

Those who are in the business try to compensate by comparing the
results of the exit polls with the actual numbers reported from early
precincts. If the margins in the exit polls are large enough (see Sen.
Clinton's run in NY), this caution is not needed. But if things are
close, this caution is absolutely necessary. At any rate, one discovers
by this empirical comparison how to "discount" the polls. The debacle of
the Florida exit polls in 2000 was caused by a simple error in
calculation. The precinct data that was used as a benchmark to judge the
accuracy of the polls was erroneous, since addition errors were made in
reporting these early totals.

By the way, the "tilt" in exit polls differs from election to election,
region to region, in ways that are not well understood.

I went to a presentation at the American Statistical Association on
this matter after the 2000 election. One of the speakers, an employee of
Voter News Service (the group that had egg on their collective faces),
began with two power point slides that read: first slide, "If you make
your living using probability theory"; second slide, "you are taking a
chance."

I hope this answers the question,  ............    Lash /



Lewis Henry LaRue
Washington and Lee University
School of Law
Lexington,  VA  24450-0303

email: laruel@wlu.edu
phone: 540-458-8513
fax: 540-458-8488

"Volokh, Eugene" <VOLOKH@law.ucla.edu> 11/3/2004 12:19:42 PM >>>

	What are the typical margins of error (that is to say, the
purely mathematical ones) and sample sizes in exit polls?  Thanks,

	Eugene