On Polls and Margins of Error

Last week I posted a retrospective on swing state polls. However, I excluded Nevada and Arizona as both states still had a significant number of votes to count. Since then, both states have gotten to 99%, so I figured I could include them. I also decided I could probably go back a bit further and looked at all polls going back to September 23rd, when the first early voting started.

This might sound bad, but I would argue it’s at least a step in the right direction. Margin of errors are supposed to represent 95% confidence intervals. With 101 polls analyzed, we would expect around 5 of them to be outside of the margin of error, but only 1 was. So that indicates there probably is some herding going on. And they still show a bias against Trump, which may be due in part to the above mentioned herding.

The one poll that outside the margin of error was a BullFinch Group Michigan poll that had Harris up by 8 percentage points and a margin of error of 4 percentage points. Remember margins of error refer to the error of a particular response; the margin of error of the gap between candidates would be twice that. So this wasn’t that far off. And the pollster specifically called out the Michigan poll as looking like an outlier. They also point out that they specifically try to avoid herding, so I would argue this is a point in favor of this particular poll.

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