Partisan Bias in Internal Polls: Lessons from the 2010 Midterm

The reliability of so-called "internal polls" released by political campaigns and parties is the subject of ongoing debate. Nate Silver has cautioned that such polls should not be taken at face value since they are "almost always designed to drive a media narrative." Silver notes that the topline results can be manipulated by several tricks of the polling trade, such as asking leading questions, or applying implausible likely voter models or demographic weightings. From analysis of such surveys , Silver has devised a rule of thumb under which he subtracts six points from the topline result of the reporting candidate or party. Thus, a poll commissioned by a Democratic candidate showing a race tied translates, under Silver's treatment, into a six-point lead for his or her Republican opponent.

In a similar vein, Mark Blumenthal has stated that "[i]t is always sensible to treat sponsored, internal surveys with extra skepticism when they are publicly released." In support of this view, Blumenthal cites studies by political scientists finding that partisan surveys show an average bias of two to four percentage points favoring the sponsoring party. Blumenthal identifies an additional reason for partisan bias in published internal polls, namely, most internals never see the light of day since campaigns typically choose to share only those polls showing good news for their candidate.

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