Nate silver's view of the amount by which different polling firms lean Democratic or Republican.
Calculating ‘House Effects’ of Polling Firms By NATE SILVER
One of the ways that our forecast model seeks to make polling data more robust is by calculating and adjusting for “house effects,” which are systematic tendencies for polling firms to favor either the Democratic or Republican candidate.
Fairly often, what is perceived as a change in the polls is really just a reflection of these tendencies. A polling firm that tends to show favorable results for Barack Obama might survey a state one week, showing him three percentage points ahead. A different polling firm that usually has favorable results for Mitt Romney comes out with a survey in the same state the next week, showing him up by two points. Rather than indicating a change in voter preferences, these results may just reflect the different techniques and assumptions that the polling firms have applied.
The chart below reflects our model’s current estimate of the house effects for some of the more prolific polling firms. (A polling firm with a Democratic house effect tends to show better numbers for Mr. Obama than in comparable surveys, while one with a Republican house effect tends to have good numbers for Mr. Romney.)
As you can see, there is a fairly wide spread in the polls this year. For instance, the firm Public Policy Polling shows results that are about three percentage points more favorable to Mr. Obama than the consensus of surveys. (How is the consensus determined? More about that in a moment.) Conversely, Gallup’s polls have been leaning Republican this year, by about 2.5 percentage points. (See the Huffington Post’s Mark Blumenthal for a good explanation as to why.)
Rasmussen Reports, which has had Republican-leaning results in the past, does so again this year. However, the tendency is not very strong – a Republican lean of about 1.3 points.
If you are used to looking at Rasmussen Reports polls, your impression may be that their partisan lean is stronger than that. This is not wrong, actually. However, some of the difference results from the fact that Rasmussen is polling likely voters, while many other polling firms are polling registered voters.
Continues at: fivethirtyeight.blogs.nytimes.com |
