Thursday, August 17, 2006

Why the "generic ballot" means nothing

Following is a great analysis from Jay Cost, a doctoral candidate of political science at the University of Chicago. From Cost:
In May, I authored an essay that took issue with the use that pundits make of the generic ballot. I made two points. The first was that pundits should be cautious in their use of it because, over its history, it has sported a large, sustained Democratic skew. This makes it quite possible to find a Democratic false positive - which pundits have managed to find in, by my count, seven of the last eight House elections. My second point was that - skew or no skew - the ballot has problems that should make us wary of it. It does not predict final results nearly as well as it should, so we should be skeptical about following it as far as it will take us.

As the use of the generic ballot among pundits persists, I thought it time to amplify my remarks - and strongly encourage them to back away from its use until closer to Election Day.
Let us start with the skew in the generic ballot.

What do I mean by "the skew?" I mean the following: the generic ballot persistently overestimates the size of Democratic support and persistently underestimates the size of Republican support. That means that pundits, insofar as they are relying upon the generic ballot, follow suit. However, we do not have to. If we have a reasonable expectation of what that skew will be - we can make a prediction about what will occur in November. All we have to do is subtract the amount of inflation from the Democratic majority.

Currently - the average June/July Gallup generic ballot of "national adults" shows the Democrats leading the GOP 51.8% to 38.4%. If we take only the people who are registering a party preference (what is known as the "two-party vote"), we can see that the Gallup generic ballot shows the Democrats leading 57.4% to 42.6% among people who prefer either the Democrats or the Republicans. That amounts to a very hefty 14.8% lead.

But this does not factor in the skew.

Historically speaking, when the Democrats have that kind of edge in June/July, by November their victory in the popular vote "shrinks" to a much more modest 51.75% to 48.25%.

In other words, today's Gallup generic ballot does not predict a Democratic blow-out. Not at all. It predicts another squeaker on the order of Bush v. Kerry. Bush's share of the two-party vote in 2004 was 51.2%. Kerry's was 48.8%. Michael Barone's "49-49 Nation," if you believe the generic ballot, has not actually gone anywhere. This year will be Round 3.
A lot of statistical mumbo-jumbo follows, which makes for reading that some will find interesting and others will find boring. Read it if you like (I did). Anywho, the conclusion:
We know two things about non-voters that are relevant for this discussion - they are more inclined to the Democrats than voters are, and they know less about politics than voters do. This could make all the difference.

Suppose, for instance, that the national mood favors the Democrats. This would induce both voters and non-voters to move into the Democratic column when queried by a pollster - but if the non-voters move into the Democratic column at an exponentially higher margin, the rate of overestimation would increase. If 3 voters and 9 non-voters moved into the Democratic column in one year, and 4 voters and 16 non-voters moved into the Democratic column in another year - the overestimation of Democratic gains would not be the same between the two years. It would go from 200% to 300%. Also, suppose that the rate of non-voter support for the Democrats, even though it is higher than the rate of voter support, varies much more dramatically because non-voters pay less attention to politics. In some years, 9 non-voters support the Democrats for every 3 voters; in other years, 27 non-voters support the Democrats for every 3 voters. That would make the extent of overestimation bounce around quite a bit. The more likely non-voters are to be in the mix, the more likely we will see the overestimation vary.

In other words - the fact that non-voters are registering support for the Democrats might be what makes the generic ballot an invalid measure. Not just skewed. Not just pro-Democratic. Not just in need of a slight corrective. But invalid. As in - don't-trust-it-because-it-will-shoot-you-and-your-dog-and-leave-you-both-for-dead invalid.
One thing we do know by now: the summer Gallup generic of adults, absent some kind of corrective measure, is not valid for making election predictions. We should stay away. It is like Australian table wine - which, as Eric Idle once noted, "is not a wine for drinking. It is a wine for laying down and avoiding!"
We all know by now how the AP polls have been oversampling Democrats by roughly 12 - 13%. This new analysis casts an even bigger cloud over the use of the generic "Which party do you prefer?" ballot questions. You would think that with these kinds of generic polls getting it so wrong every time, the MSM pollsters would have learned their lesson by now. You would think...and therein lies the pollsters' problem!