Right, but 538 did account for such an error in coming up with their ~75/25% outcome chances. Had polling errors just been random, the overall prediction would have been more like 95/5%.

But their key insight is that polling errors are often correlated, and not just statistical sampling errors that can randomly offset in either direction.

> The +/- in a given poll is measurement error derived from the sample size. They don’t account for systemic error.

Sure, but even the worst poll-based prediction outfit isn't just aggregating poll results and using the uncertainty based on sample size to determine the probabilities of different outcomes.

Pretty much all of them are using some model derived from the past relationships of polls to election results, which will, to a greater or lesser extent, capture systemic polling error that is not unique to the current cycle in the model.

19/20 does, though.

What is 19/20?

Polls are reported with fine print similar to "+/- 2%, 19 times out of 20".