But the U.S.C./LAT poll is a panel — which means it recontacts the same voters over and over — and so it wound up with the worst of both worlds.

If the U.S.C./LAT poll were a normal poll, the 19-year-old from Illinois might have been in the poll only once. Most of the time, the heavily weighted young black voters would lean toward Mrs. Clinton — ensuring that the poll both had the appropriate number of black voters, and a relatively representative result.

But the U.S.C./LAT poll had terrible luck: The single most overweighted person in the survey was unrepresentative of his demographic group. The people running the poll basically got stuck at the extreme of the added variance.

By design, the U.S.C./LAT poll is stuck with the respondents it has. If it had a slightly too Republican sample from the start — and it seems it did, regardless of weighting — there was little it could do about it.