--Florida moves from "Toss up" to "Solid Republican"
--Kentucky moves from "Toss-up" to "slight lean Republican"
--Missouri moves from "Toss-up" to "slight lean Democrat"
--Ohio moves from "Toss-up" to "slight lean Republican"
--Pennsylvania moves from "Toss-up" to "slight lean Republican"
States with polling changes: Arkansas, California, Connecticut, Florida, Kentucky, Missouri, Nevada, North Carolina, Ohio, Pennsylvania, Wisconsin
Commentary: The improvement in the Republican position comes from refinements to the "toss-up" category. Rather than all "toss-ups" being given 50-50 chances of going to one party or another, they have been weighted in favor of the part with the slight advantage. Republicans are now projected to win five of the seven campaigns that were formerly listed as true "toss-ups." This improves their overall standing to a net gain of five seats. I believe this more accurately reflects the current political situation.
Specific percentages for each overall partisan result (a 28% chance for 58 Democratic seats, a 30% chance for 57 Democratic seats, a 25% chance for 56 Semocratic seats, etc) will become available in mid-2010, once more polling is available for each campaign, and once most primary campaigns are decided. The forecast will always use the plurality percentage as the topline result.
Republican-Held Seats Democratic Pickups: 1 (One of Kentucky, Missouri or Ohio)
For now, use the simple mean of all polls where the majority of interviews were conducted since September 30th (the last 90 days).
When available, at least two polls are used for every campaign, even if the majority of their interviews were conducted before September 30th.
As the election draws closer, restrict the timeframe for polls included in the averages. During the final six weeks of an election, use the simple mean from the last 15 days.
In campaigns with more than one poll, those where one party is ahead by 6.0 or more are considered "solid." Campaigns between 2.7 and 5.9 are considered "leans." Campaigns within 2.6 or less are considered "toss-ups." These categories are subject to refinement based on continued research into past elections.
"Solid" and "lean" seats are considered pickups, while toss-ups are 50-50 for each party. States with mixed results will be considered 50-50 until the primary election. The overall forecast is the most likely seat change based on the current forecast. This is also subject to refinement.
Include campaign-funded polls. Further, if there is more than one poll from a single organization, include all of them.
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.
*= Faces primary, but heavy favorite
**= Not an announced candidate at this time