Don’t Trust California’s Exit Polls [CORRECTED]

CORRECTION Feb. 6, 1:05 p.m.: Well, I was wrong. Contrary to my original post, California’s exit poll data did include a sample of early voters polled via the phone. From Associated Press’ account of the methodology :

Therewere 17,454 interviews of Democratic primary voters, and 11,205 GOP voters.Results included 1,005 telephone interviews of Democratic absentee voters and813 GOP absentee voters in Arizona, California and Tennessee.Overall sampling error was plus or minus 1 percentage points for both parties.

We gleaned our information from CNN, which has a misleading exit poll description of its own. What CNN says about their exit polls :

Exit polls are a survey of selected voters taken soon after they leavetheir voting place. Pollsters use this sample information, collectedfrom a small percentage of voters, to track and project how all votersor a specific segments of the voters sided on a particular race orballot measure.

No mention of an early voter phone survey. The entire country uses the same exit poll data, which means CNN uses the same data the AP uses, both from Edison Media Research. I spoke to someone in their office who confirmed the AP’s account that a poll was conducted by phone with early voters. We regret the error. The original piece is below.

This item was cross-posted at Slate ‘s Election Scorecard .

Poll junkies beware: California exit polls are not to be trusted.

Californiahas issued 5.5 million absentee ballots for today’s primary, reachingmore than one-third of the 15.7 million total voters registered in thestate. As of yesterday, 3 million ballots had already been returned,and state officials expect about 75 percent of the ballots to bereturned by the close of polls—that’s 4.125 million people who votedwithout pulling a lever. (These numbers include both Democratic andRepublican ballots.) The remaining ballots are expected to be turned inat polling stations today, just like you drop off a movie rental.

Forour purposes, it’s the 3 million ballots that have already been sentback that may play havoc with expectations tonight. Exit polls, astheir name implies, measure only the opinions of residents who go tothe polls and submit a ballot. If you don’t show up to the votingbooth, you’re not going to be part of an exit poll.

Conventional wisdom suggests early voters chose Hillary Clinton over Barack Obama, since Clinton led in the polls until recently. In the most recent SurveyUSA poll ,34 percent of respondents told the pollster they had already voted,which echoes the predicted vote-by-mail participation rate. Of thosevoters, 54 percent favored Clinton and 37 percent favored Obama. Weshould note that SurveyUSA shows a stronger Clinton lead than otherpolls ( Zogby , Rasmussen ) released in the last 48 hours.Givenall this, exit polls that show a slight lead for Obama actually may bebad news for Barack. If SurveyUSA’s poll is correct—no sure thing—thenObama will need a strong lead (around 10 percent) among today’s votersto win California.