Nate Silver and 538’s Measurable 3.5% Democratic Bias and the 2018 House Race

Since beginning to adjust or unskew polls for house effect during the 2014 United States Senate elections, Nate Silver and FiveThirtyEight (538) have a measurable 3.5% bias toward Democrats. This 3.5% bias is found by taking the final 538 projected gap between the Democratic and the Republican candidate in 125 races over the 2014 and 2016 election cycles and comparing that projected gap to actual final results. (The raw numbers for comparison can be found here. I am grateful to a professor in Britain, who works with statistics but wishes to remain anonymous, for reviewing the data for errors.)

A portion of the 3.5% 538 bias in the most recent elections can be attributed to normal polling margins of error that have over-projected Democrats, generally within the margin of error, during those two cycles. RealClearPolitics (RCP), for example, had a 1.7% statistical bias in favor of Democrats for the 2016 cycle in Senate and Presidential races it projected. This compares to 538’s 3.3% Democratic bias for the 2016 cycle in Senate and Presidential results.* For the battle to control the 2018 U.S. House of Representatives, 538’s projection is firmly in the #BlueWave camp, regularly projecting a “Democratic seats average gain” of well over 30, sometimes nearing 40. A thirty seat gain puts Democrats into #BlueWave territory with 225 or more seats, giving the party comfortable control of the House while leaving Republicans with 210 or fewer seats.

Our modeling…

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