https://fivethirtyeight.com/features/the-state-of-the-polls-2019/
«As a simple rule of thumb, we’ve found polls “call” the right winner 80 percent of the time, meaning they fail to do so the other 20 percent of the time….
Polls in our database that showed a lead of 3 percentage points or less identified the winner only 58 percent of the time — a bit better than random chance, but not much better.»
