The big change in this forecast is the inclusion of win percentages for each campaign. The win percentage is based on if the election were held today. The specific percentage is based on my research into the 138 closest statewide campaigns from 2004-2009. It projects the likelihood of a Democratic victory given the current polling average.
In campaigns where the general election matchup has not been decided, the win percentage used in the overall forecast features the two candidates I believe will win their respective primaries. That number can be seen in boldface in the win % column. The number is usually based on polling, but is not entirely based on polling. For example, I believe that Rubio will be the Republican nominee in Florida, even though polling says otherwise right now.
Overall, the inclusion of win percentages has very slightly improved the Democratic position in the forecast. Two weeks ago, I projected Democrats would have 55 Senators in 2011-2012. Today, I project 56.
With the exception of primary elections, the forecast is entirely based on polling.
For now, use the simple mean of all polls where the majority of interviews were conducted since October 14th (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 October 12th.
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" (95% chance for leading party to win). Campaigns between 1.8 and 5.9 are considered "leans" (75-95% chance for leading party to win). Campaigns within 1.7 or less are considered "toss-ups."
The win % represents the chance for a Democratic victory given the current polling averages. It is based on my research into the 138 closest statewide campaigns from 2004-2009. For example, across those 138 campaigns, there is a 50% chance for the final polling average to differ from the final result by more than 1.7%. Thus, a candidate ahead by 1.7% is given a 75% chance of winning, since the polling average has an equal chance of being skewed in favor of either major-party candidate.
In campaigns where the general election matchup has not been decided, the win percentage used in the overall forecast features the two candidates I believe will win their respective primaries. My projected primary winners are indicated by the win percentage for their general matchup appearing in boldface. My primary picks are mainly based on polling, but not entirely on polling. Factors such as money, name ID, and activist excitement are also taken into account. Truthfully, I have no systematic means for forecasting primary elections, given the paucity of data and poor performance of past primary polling.
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.
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.
Notes:
*= Faces primary, but heavy favorite
**= Not an announced candidate at this time
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