So which is it ? Biden ahead in battlegrounds or Trump ?

[BGBD]

18. that's

my point.

If anyone watched the Upshot polls last cycle, you could see one just starting with 50 responses and a candidate could be up 10 or 15 points, but by the time the poll was complete that same candidate was down 5 or more.

Polls was a function if random sampling and you need a large enough n for the central limit theorem to take effect. If I talk to 10 people at random, there's a good chance in a dead even race that I happen to talk to a couple more of one supporter than the other, and my results are 70/30. I could conclude that one candidate is way ahead, or I could talk to another 490.