National House Ballot Update, December 16th

by: Chris Bowers

Wed Dec 16, 2009 at 12:09

December 16th: Democrats +0.4%

Last update: Dec 15: Democrats +0.5%

Changes in the polls:
--Dec 08 GWU added

Recent results:
2008: Democrats +8.9% (257 seats)
2006: Democrats +7.9% (233 seats)
2004: Republicans +2.6% (203 seats)
2002: Republicans +4.6% (206 seats)

Polls included in the calculation

Poll Sponsor Poll Mid-Date Democrats Republicans

Total Dec 16 39.57 39.21

Gallup Dec 12 48 45

Rasmussen Dec 11 37 44

Daily Kos Dec 09 37 33

GWU Dec 08 41 42

Economist Dec 07 46 36

PPP Dec 06 44 42

Bloomberg Dec 05 38 42

CNN Dec 03 40 39

Rasmussen Dec 03 39 43

Daily Kos Dec 02 36 32

Rasmussen Nov 26 37 44

Daily Kos Nov 24 37 32

Rasmussen Nov 20 37 44

Daily Kos Nov 18 37 31

Chris Bowers :: National House Ballot Update, December 16th

Methodology:

Until the final six weeks of the election, take the simple mean of virtually all polls where the majority of interviews were conducted over the last 30 days.
During the final six weeks of an election, use the simple mean from the last 15 days.
Do not include Zogby Interactive polls and Columbus Dispatch polls, due to their horrendous past performance and questionable methodologies.
Do not include Strategic Vision polls, as it is starting to seem likely those are not real polls.
Include campaign-funded polls. Further, if there is more than one poll from a single organization, include all of them.
In the interests of a smoother average, polling outfits that release new generic ballot polls every week, such as Rasmussen and Daily Kos, will always have the same number of polls in the average.

The basic idea is to cram as many polls with sound methodologies into the averages as possible, and weight them evenly to include more overall data in the sample. Because voter preferences don't really change that much in high-profile elections, I thought this method might produce a more accurate result through logic of regression to the mean. It seems to work pretty well, as my research has shown so far.

This is different from my 2006 and 2008 methodology in that it includes polls from 15 days out from an election, instead of only 8. Further, campaign funded polls, and multiple polls from a single polling firm, are now included. All of these changes were made to include more polls in the averages, since my previous methodology was about 10-20% less accurate than Pollster.com and fivethrityeight.com. Since they had already raised the bar so high, and since they will probably improve their methodologies for 2010 even more, it was time for Open Left to step it up.