September 9, 2010

Gallup 'unexpectedly' reverts to no GOP lead?

Wow, a net 10 point swing in one week.  Gallup now has the GOP and the Democrats tied at 46% support after the GOP lead of 51% to 41% previously. What happened? Game over for the GOP? They blew it?


From Gallup's footnotes (emphasis added):
Results are based on telephone interviews conducted as part of Gallup Daily tracking survey Aug. 30-Sept. 5, 2010, with a random sample of 1,651 registered voters, aged 18 and older, living in all 50 U.S. states and the District of Columbia, selected using random-digit-dial sampling.
For results based on the total sample of registered voters, one can say with 95% confidence that the maximum margin of sampling error is ±4 percentage points.
Interviews are conducted with respondents on landline telephones and cellular phones, with interviews conducted in Spanish for respondents who are primarily Spanish-speaking. Each daily sample includes a minimum quota of 150 cell phone respondents and 850 landline respondents, with additional minimum quotas among landline respondents for gender within region. Landline respondents are chosen at random within each household on the basis of which member had the most recent birthday.
Samples are weighted by gender, age, race, Hispanic ethnicity, education, region, adults in the household, cell phone-only status, cell phone-mostly status, and phone lines. Demographic weighting targets are based on the March 2009 Current Population Survey figures for the aged 18 and older non-institutionalized population living in U.S. telephone households. All reported margins of sampling error include the computed design effects for weighting and sample design.
In addition to sampling error, question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of public opinion polls.
First of all it's registered voters and not likely voters. But for Gallup, that's apples-to-apples. While last week I argued apparently forgot to argue that 10 points was an overstatement of the GOP advantage for Gallup's registered voters methodology, so too, this is an understatement of the GOP lead. Both week's are likely outliers in the Gallup results.  Since early August the GOP has ranged from 46% to 51% with an approximate average of 49%.  Democrats have ranged from 41% to 46% with an approximate average of 43.5%.  Overall that's an average GOP lead of 5.5%.

There's a +/- variance of 4% on the poll too. That means instead of 46/46 it could be 50/42 for Republicans, or for that matter for the Democrats. Last week's 51/41 in favor of the GOP could have been as high as 55/37 or as low as 47/45.  One day's sample's worth of data does not a trend or preponderance of evidence make.  Not entirely coincidentally, the average generic ballot numbers over that same period (early August through now) falls within the margin of error of both the latest poll and last week's as well.

There's no mention of weighting based on party affiliation, but that could be a factor as well. Furthermore, Gallup is still using registered voters.  Now is the time pollsters usually start switching to likely voter models. Backing out possible media bias to try to spin things for the Democrats, the GOP wave still looms large. I expect if Gallup sticks with the registered voter model that next week we'll see something like GOP 48% and Democrats at 43%.  In other words, Republicans, conservatives, anti-big-government advocates, do not panic. If you need further assurance, Rasmussen has it here.

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