Decoding the National Polls

From the Center for American Progress and The Century Foundation:

In this edition of Public Opinion Watch:

(covering polls and related articles from the weeks of September 13-19, 2004)

• Decoding the Gallup and New York Times Polls
• Persuadable Voters Still Not Persuaded

Decoding the Gallup and New York Times Polls

Here are Bush's leads in the three national polls released before Gallup's current poll (no registered voter [RV] data available for Democracy Corps and Harris, only likely voter [LV] data; Pew and Harris matchups include Nader):

Democracy Corps, September 12-14 LVs: +1

Pew Research Center, September 11-14 RVs: tied

Harris Interactive: September 9-13 LVs: -1

Looks like a tie ball game, right? But according to the Gallup poll conducted September 13-15 and released September 17, Bush is up... by thirteen points??

Let's just say I'm just a wee bit skeptical of this one. First, Gallup's poll only includes one day (September 15) that the three other polls do not cover, so it can't be Gallup's survey dates that explain the big Bush lead.

Second, this thirteen-point lead is an LV figure and, as I've repeatedly emphasized, Gallup's LV screening procedure produces completely untrustworthy measures of voter sentiment this far in advance of the election. Here is a summary of the case against Gallup's LV data:

Sampling likely voters is a technique Gallup developed to measure voter sentiment on the eve of an election and predict the outcome, not to track voter sentiment weeks and months before the actual election. There is simply no evidence, and no good reason to believe, that it works well for the latter purpose. In fact, the evidence and compelling arguments are on the other side: that the registered voters are the more reliable gauge of voter sentiment during the course of the campaign.

Here's why. Gallup decides who likely voters are based on seven questions about their interest in voting, attention to the campaign, and knowledge about how to vote (e.g., where their polling place is located). The interested/attentive/knowledgeable voters are designated "likely" and the rest are thrown out of the sample. But as a campaign progresses, the level of interest among voters tends to change, particularly among those with partisan inclinations whose interest level will rise when their party seems to be mobilized and doing well and fall when it is not. Because of this, partisans of the mobilized party (lately, Republicans) tend to be screened into the likely voter sample and partisans of the demobilized party (lately, Democrats) tend to get screened out. But tomorrow, of course, the Democrats could surge, in which case their partisans may be the ones over-represented in likely voter samples.

That suggests the uncomfortable possibility that observed changes in the sentiments of "likely voters" represent not actual changes in voter sentiment, but rather changes in the composition of likely voter samples as political enthusiasm waxes and wanes among the different parties' supporters. And that is exactly what political scientists Robert Erikson, Costas Panagopoulos, and Christopher Wlezien find in their analysis of Gallup's 2000 RV/LV data in their forthcoming paper, "Likely (and Unlikely) Voters and the Assessment of Campaign Dynamics" in Public Opinion Quarterly: "shifts in voter classification as likely or unlikely account for more observed change in the preferences of likely voters than do actual changes in voters' candidate preferences."

That means that, instead of giving you a better picture of voter sentiment and how it is changing than conventional registered voter data, likely voter data give you a worse one since true changes in voter sentiment are swamped by changes in who is classified as a likely voter.

I think the case against the Gallup LV data looks rock solid. In my view, it's time for them to drop reporting these data because they are highly likely to give an inaccurate picture of the state of the race and, by doing so – especially given the high profile of Gallup's polls – unfairly pump up one side of the race and demoralize the other. That doesn't seem acceptable to me.