Some polls about recent primary races in several states have been off. In a report from ABC, some of the difficulties in predicting primary elections are discussed:

Experts say this year’s primaries are highlighting some of the pitfalls of political polling.

“As a general rule, primaries are much harder to predict than general elections,” said Brad Coker, managing director of Mason-Dixon Polling and Research. “The hard part is figuring out who’s going to show up.”

Pollsters say developing a fundamental sense of who is going to vote is harder to do in primary elections when turnout is historically lower and more variable. There’s also the early and absentee voting factor.

This sounds like a sampling issue. Political polls tend to search for people who are likely to vote. But if a large percent of the sample that pollsters reach aren’t going to vote, the results are not very trustworthy.