Steve Ansolabehere and Jim Snyder estimate the incumbency advantage in the past set of House elections to be 5 percentage points. They estimated these the week of the election and then re-estimated once most of the states produced their certified votes, about a month later, and they seem consistent with our post-election estimate of 6 percentage points, which is a bit lower than the 8-10 percentage-point advantage that was the norm in the 1980s and 1990s. As Steve writes, the declining incumbency advantage is one part of the story by which the Republicans were able to grab so many seats in 2010.
