Guest Pollster | November 7, 2008
Today's Guest Pollster contribution comes from Mark DiCamillo, director of The Field Poll in California.
Having put to rest the so-called Bradley effect in this year's presidential election, we are now seeing numerous references to a so-called "Bradley effect" regarding the California vote on Proposition 8, the same sex marriage ban. The Bradley effect in the California gubernatorial election, even back in 1982 was minimal (at most 2 pts out of the 8 point error in the pre-election polls). It was a convenient theory for people to use when describing the fallibility of the pre-election polls conducted in California in that year, but a closer examination would find most of the polling errors were not due to factors relating to racial bias.
While the notion that social desirability effects could have played a role on a controversial social issue like same-sex marriage, tit s theory without any real evidence, whereas an alternate explanation of the deviation between the pre-election polls and the election outcome is far more compelling and is supported by the data.
First, a quick review of the pre-election polling done by the two leading public opinion polls in California, The Field Poll and the Public Policy Institute of California. They show the following trend:
- July Field: No = +9
- August PPIC: No = +14
- Early Sept Field: No = +14 or +17 (depending on wording)
- Mid-Sept PPIC: No = +14
- Mid October PPIC: NO = +8
- Late October Field: No= +5
- Election outcome: Yes = +4
These data show the No side ahead by double- digit margins throughout most of the pre-television campaign stages. However, as the TV advertising hit in mid to late September, the Yes campaign ads proved to be more effective, and the polls showing the No side advantage slipping.
The movement continued into and through the final weekend of the election when the churches and various religious groups made a concerted effort to win over the support of their congregations. The evidence shows that they were successful.
When comparing the findings from The Field Poll's final pre-election survey of likely voters (n-966) to the Edison Media Research exit poll in California, the biggest differences relate to the turnout and preferences of frequent church-goers and Catholics. The Field Poll, completed one week before the election, had Catholics voting at about their registered voter population size (24% of the electorate) with voting preferences similar to those of the overall electorate, with 44% on the Yes side. However the network exit poll shows that they accounted for 30% of the CA electorate and had 64% of them voting Yes. Regular churchgoers showed a similar movement toward the Yes side. The pre-election Field Poll showed 72% of these voters voting Yes, while the exit poll showed that 84% of them voted Yes.
The same kind of phenomenon occurred when the first same-sex marriage ban was voted in California in the March 2000 election (Prop. 22), although because of the size of its victory( 61% Yes vs. 39% No) it didn't matter much back then. In that year The Field Poll's final pre-election poll, also completed about one week prior to the election, had 50% of Catholics on the Yes side, and accounting for 24% of the vote. Yet, the network exit poll conducted that year by Voter News Service showed them to account for 26% of the electorate with 62% voting Yes.
My take is that polling on issues like same-sex marriage that have a direct bearing on religious doctrine can be affected in a big way in the final weekend by last minute appeals by the clergy and religious organizations.