One thing I found interesting about the final polling averages was that they were much more correct about Biden's share of the vote, even when they were very wrong about the margin itself (since these polls don't add to 100%, that's possible). See below for the 538 polling average vs. the current NYT tally (% for Biden):
PA: 50.2% vs 49.7%
FL: 49.1% vs 47.8%
TX: 47.4% vs 46.3%
OH: 46.8% vs 45.2%
IA: 46.3% vs 44.9%
NC: 48.9% vs 48.6%
WI: 52.1% vs 49.5%
MI: 51.2% vs 50.5%
GA: 48.5% vs 49.3%
AZ: 48.7% vs 48.9%
NV: 49.7% vs 49.9%
Most are within ~1% or so (some for Biden, some for Trump) with some outliers being Wisconsin (-2.6%), Ohio (-1.6%) and Iowa (-1.4%) in Trump's favor still not being so far off (relative to margins of error).
I'm not sure what that "missing" bit in the polls really means (undecided?) but it seems the issue was that that bit of the electorate ended up going entirely for Trump in many places.
That's really interesting, I hadn't sliced it that way yet.
I know FiveThirtyEight allocates undecideds evenly - if 8% respond undecided they assume 4% will break Biden and 4% will break Trump. In one of the podcasts they discussed this assumption and it's what they have found to be the most accurate historically.
We might be seeing a shift towards that no longer being true - maybe 75% break red and 25% break blue now.
Edit: Also, It looks like Jorgensen (Libertarian) significantly underperformed her polling (~3% > ~1%) in the first few states I spot checked. If those voters broke for Trump, that makes up about 2% of the error. That'd take a lot more in depth checking though.
Probably worth noting that the asymmetric break of the undecideds in 2016 was what won the election for Trump. There were a lot of undecided voters and a lot of them decided for Trump late in the process so models that assumed a traditional 50-50 split were wrong.
PA: 50.2% vs 49.7%
FL: 49.1% vs 47.8%
TX: 47.4% vs 46.3%
OH: 46.8% vs 45.2%
IA: 46.3% vs 44.9%
NC: 48.9% vs 48.6%
WI: 52.1% vs 49.5%
MI: 51.2% vs 50.5%
GA: 48.5% vs 49.3%
AZ: 48.7% vs 48.9%
NV: 49.7% vs 49.9%
Most are within ~1% or so (some for Biden, some for Trump) with some outliers being Wisconsin (-2.6%), Ohio (-1.6%) and Iowa (-1.4%) in Trump's favor still not being so far off (relative to margins of error).
I'm not sure what that "missing" bit in the polls really means (undecided?) but it seems the issue was that that bit of the electorate ended up going entirely for Trump in many places.
(edit: had the PA 538 # wrong)