Updates to the simple trends and house effects models for all 190 national likely-voter polls released as of noon on Monday, November 5th and archived at Huffington Post Pollster.
Polling Methods
If we include only interviewing methods and exclude the pollster house effect dummies, we find a significant pro-Republican effect for automated methods and an modest pro-Democratic effect for Internet polling. However the results for automated polling are really just a proxy for Rasmussen. When a dummy variable for that firm is included, the effect for automated polling becomes insignificant. Other firms like PPP that employ automated interviewing show no significant biases in either direction.
175 Likely Voter Polls from Pollster (7/1-11/4)
Dependent variable: Obama Lead over Romney
Ordinary Least Squares
coefficient std. error t-ratio p-value
----------------------------------------------------------
const 3.66461 0.637713 5.746 4.19e-08 ***
DaysBefore −0.0194144 0.0102005 −1.903 0.0587 *
Debate1 −3.40771 0.570555 −5.973 1.35e-08 ***
Sandy 0.969663 0.727894 1.332 0.1846
Internet 1.17270 0.533334 2.199 0.0293 **
Auto −0.140361 0.662144 −0.2120 0.8324
Rasmussen −2.46427 0.760485 −3.240 0.0014 ***
Mean dependent var 0.965714 S.D. dependent var 2.866567
Sum squared resid 936.3252 S.E. of regression 2.360797
R-squared 0.345133 Adjusted R-squared 0.321745
F(6, 168) 14.75677 P-value(F) 1.64e-13
The same cannot be said for Internet polling. Including or excluding the dummy variables for individual firms that use Internet polling does not eliminate the effect for that method. On the average, firms that use Internet polling report results about one percent more Democratic than firms who use human interviewers. The final model below keeps the Internet effect but excludes the measure for automated polling.
175 Likely Voter Polls from Pollster (7/1-11/4)
Dependent variable: Obama Lead over Romney
Ordinary Least Squares
coefficient std. error t-ratio p-value
--------------------------------------------------------------
const 3.77159 0.602485 6.260 3.17e-09 ***
DaysBefore −0.0211743 0.00981172 −2.158 0.0324 **
Debate1 −3.21365 0.549629 −5.847 2.59e-08 ***
Sandy 0.535302 0.701852 0.7627 0.4467
Internet 1.05117 0.508563 2.067 0.0403 **
Rasmussen −2.63924 0.467508 −5.645 6.99e-08 ***
DemocracyCorps 2.05705 1.04816 1.963 0.0514 *
ARG −1.54940 0.885694 −1.749 0.0821 *
Gallup −4.55248 1.33905 −3.400 0.0008 ***
Mean dependent var 0.965714 S.D. dependent var 2.866567
Sum squared resid 842.2114 S.E. of regression 2.252457
R-squared 0.410956 Adjusted R-squared 0.382569
F(8, 166) 14.47659 P-value(F) 6.25e-16
House Effects
I have been much stricter in my criteria for house effects in this final model. I have only included these effects if they are significant at the 0.10 level and are based on more than one poll. Organizations that released a single poll in our timeframe like Reason/Rupe are not singled out in this model. That leaves five firms with identifiable biases. Gallup, Rasmussen and ARG all tilt Republican while DemocracyCorps and, especially, Bloomberg show Democratic biases. The Bloomberg result is based on just two polls and may not represent a systematic house effect. The Gallup result is based on the three tracking poll reports after the firm switched to using its likely-voter model. Gallup stopped interviewing after Hurricane Sandy so there are no polls from that firm more recent than October 28th.
Trends and Events
No trend changes are seen after the first Presidential debate nor after Hurricane Sandy. Including “interaction” terms for the product of DaysBefore with these two variables produce insignificant results. The overall effect of the debate was to reduce the President’s estimated Election Day margin in polling from 3.8% to 0.6%. Polling since Hurricane Sandy shows a modest 0.5% increase in support for the President, but it fails to achieve statistical significance. However the pace at which President Obama gains support over time falls to its lowest overall level in this model. Over the course of the campaign the President increased his margin of victory by one percentage point every 47 days (=1/0.0211743).
