141 Likely Voter Polls from Pollster (7/1-10/25) Dependent variable: Obama Lead over Romney Ordinary Least Squares coefficient std. error t-ratio p-value ------------------------------------------------------------ const 5.58618 0.59754 9.349 4.21e-16 *** Debate1 −5.58195 1.19655 −4.665 7.76e-06 *** Trends Pre-Debate −0.05086 0.00956 −5.321 4.56e-07 *** Post-Debate −0.00685 0.04817 −0.142 0.8871 House Effects Rasmussen −2.24266 0.46214 −4.853 3.53e-06 *** Gallup −4.20741 1.20723 −3.485 0.0007 *** ReasonRupe 3.95669 2.02626 1.953 0.0531 * AllstateNJ 3.85498 2.02674 1.902 0.0594 * UTechNJ −3.70445 2.03258 −1.823 0.0707 * NPR 2.82984 1.46391 1.933 0.0555 * DemCorps 2.26101 1.05003 2.153 0.0332 ** WashTimesJZA 2.05687 1.04592 1.967 0.0514 * ARG −1.82185 0.85877 −2.121 0.0358 ** Methods NetNotIpsos 2.67967 0.80298 3.337 0.0011 *** IpsosReuters 0.63428 0.65526 0.968 0.3349 Mean dependent var 1.120567 S.D. dependent var 3.078671 Sum squared resid 506.0489 S.E. of regression 2.004061 R-squared 0.618638 Adjusted R-squared 0.576264 F(14, 126) 14.59960 P-value(F) 3.00e-20
Results for polls with Rasmussen excluded (N=103)
coefficient std. error t-ratio p-value ------------------------------------------------------------ const 6.21481 0.789507 7.872 7.96e-12 *** Debate1 −5.58066 1.33422 −4.183 6.73e-05 *** Trends DaysBefore −0.0606532 0.0140282 −4.324 3.99e-05 *** DayB4*Deb1 0.0141113 0.0520640 0.2710 0.7870 House Effects Gallup −4.08319 1.16342 −3.510 0.0007 *** UTechNJ −3.97064 1.95781 −2.028 0.0455 ** ReasonRupe 3.81785 1.94636 1.962 0.0529 * AllstateNJ 3.69654 1.94702 1.899 0.0609 * NPR 2.95549 1.43444 2.060 0.0423 ** DemCorps 2.35408 1.02893 2.288 0.0245 ** WashTimesJZA 2.28335 1.01499 2.250 0.0269 ** ARG −1.76455 0.825079 −2.139 0.0352 ** Methods NetNotIpsos 2.75094 0.772406 3.562 0.0006 *** IpsosReuters 0.611675 0.629106 0.9723 0.3335 Mean dependent var 1.883495 S.D. dependent var 3.000983 Sum squared resid 329.3809 S.E. of regression 1.923775 R-squared 0.641432 Adjusted R-squared 0.589057 F(13, 89) 12.24691 P-value(F) 8.35e-15
The method for measuring pre- and post-debate trends differs in the two models. For polling with Rasmussen included, I created separate pre- and post-debate trends and tested whether each significantly differs from zero. In the model without Rasmussen, I measured whether the post-debate trend differed from the pre-debate trend by including an overall trend measure and an interaction term that tests whether the post-debate slope differs from the slope estimated using all the observations. Mathematically the tests are identical, but the estimated coefficients tell different stories. The first model shows that the trend before the Denver debate was significant but shows no trend since then. The second model shows that, in the sample without Rasmussen polls, the remaining pollsters all report results that follow the same common trend line. There is no difference between pre-debate and post-debate trends in the sample without Rasmussen polls.
