Why does Clinton do better in some polls and Sanders in others? House effect and bad methodology. [View all]

[Attorney in Texas]

House effect is where some pollsters consistently favor one candidate by more than the margin of error for the other contemporaneous polls.

For example, the most recent poll that had Clinton way up in Iowa was by Loras, which has a HUGE pro-Clinton house effect.

Here is how the race in Iowa has developed over the past year according to Loras polling:



Here is how the race in Iowa has developed over the past year according to EVERY OTHER LIVE PHONE pollster:



Clinton and her supporters set the bar of expectations based on Loras polling at their own risk (remember the fate of those who have won Iowa but fell short of expectations -- Harkin '92, Gephardt '88, etc.)

Also, all campaign season there has been an odd phenomenon that robo-call polls have consistently shown a large pro-Trump and pro-Clinton effect (I have not seen a convincing explanation for this, but the effect is well documented).

This explains why you see most traditional polls according to well-proven polling methods show Sanders leads in Iowa and New Hampshire with contemporaneous robo-call polls from Gravis and Monmouth that show Clinton ahead in Iowa and a tighter race in New Hampshire.

If you do nothing other than exclude robo-call polls from the Pollster aggregator, Sanders is ahead in Iowa and Sanders is comfortably up by double-digits in New Hampshire:





It does Clinton no favors to set her expectations in Iowa based on robo-call polls because, historically, falling short of expectations is almost worse than losing in Iowa.