What’s the actual margin of error?

Thomas Lumley over at StatsChat has used Peter Green’s polling average code to estimate the actual margin of error for political polls after adjusting for design effects. I had no idea how this could be attempted across non-probability samples (EDIT: To be fair, I had no idea how this could be attempted across multiple polls – at all).

If the perfect mathematical maximum-margin-of-error is about 3.1%, the added real-world variability turns that into about 4.2%, which isn’t that bad. This doesn’t take bias into account — if something strange is happening with undecided voters, the impact could be a lot bigger than sampling error.

That last point is a fairly important one. There are many potential sources of error in a poll other than the sampling error.

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