Are all "likely voter" models created equally? Not at all.
Case in point, the comment left yesterday by George Mason University political science Professor Michael McDonald about the latest Time/SRBI poll:
Continuing my war on likely voter models...
Here we have 808 "Registered Likely Voters." Q1 reports 100% of the sample is registered and Q2 reports 90% are "definitely" going to vote and 10% "probably." I guess this means that registered likely voters must have to respond affirmatively to being registered and "definitely" or "probably" to voting. This is different from Gallup, which requires likely voters to have a past history of voting and to express an interest in the campaign. There is no indication of weighting in this survey, so who knows what it going on there.
If I am correct, then this two-question likely voter model seems less biased against young voters and less volatile due to changing interest. This may explain the stability since June in this poll compared with the USAToday/Gallup poll.
Mike's theory seemed plausible, so I sent an email to Mark Schulman, CEO of Abt SRBI, the firm that conducts the Time poll. Here is his full response:
Mike, the Time sample is indeed weighted based upon the entire cross-section sample, as are most election surveys. We retain demographics for the entire sample, registered or not, and weight the entire cross-section sample on the usual Census demographic variables. The 100% you cite is the total of self-reported registered voters who are then asked about likelihood to vote. It does not include unregistered screen outs, who skip straight to the weighting demographics. I see that this can cause confusion. I'm glad that you requested this clarification.
You are correct in that we are not currently using past vote in our model. My objective in the pre-convention polling is to be fairly inclusive in the voter model until after the nominating conventions, when the campaigning starts in earnest. We're likely being a bit too inclusive with the light voter screen, but this still improves upon reporting based upon registered voters. Research on models which include "interest in campaign" and related questions finds variability in the composition of the likely voter profile during early campaign period, leading to some volatility in the estimate. This volatility is reduced as the election approaches.
We always tighten the model a notch after the nominating conventions. To be perfectly honest, I don't claim to have all the answers at this point on which approach we will use to tighten the model. I'm concerned about the likely influx of new voters, young voters, newly registered voters, newly activated voters. In 2004, we had an increase in turnout, even with an incumbent whose job rating was still just below 50% at that time. I don't have a fix at the moment on what to expect in 2008. Our plan is to consult with several leading experts in turnout models later this month and then make some decisions on which approach to take on our turnout model and targets. We're not wedded to any one approach. FYI, for internal purposes, we do break out our horse-race data by likelihood to vote to gauge the impact of smaller vs. larger turnouts.
I do wish to emphasize that we should not strictly abide by past turnout percentages reported by the U.S. Census. Our landline telephone universe is smaller than the Census CPS universe because of undercoverage. Therefore, our target turnout number will be higher than Census turnout trend data would suggest.
Thank you again for requesting this clarification.
If all of this detail confuses you, here is the short version: The Gallup Likely voter model, as applied to the last two USA Today/Gallup polls, uses self-reports of past voting and interest in the election to help identify "likely voters" (in addition to questions about registration and intent to vote). In surveys conducted before the conventions, the Time/SRBI poll does not -- it uses only questions about registration and intent to vote.
Update: Although the Time/SRBI poll uses a simplified likely voter model that should produce less volatility, their sample of registered voters managed to include an even smaller percentage of 18-to-29-year-olds (9% - see QF1) than the "likely voters" in the USA Today/Gallup survey (10%) discussed earlier, and six points fewer than the self-ID'd registered voters in the Gallup survey (15%).