Campaign internal polling is often more accurate than public polling. The campaigns dedicate more resources and want the polls as accurate as possible. The campaigns that take shortcuts can wind up in a world of hurt.
Media polls often do the same. They rely on pretty inaccurate online polls or they don’t call enough people or they form groups of voters to call based on databases, etc. There are a lot of shortcuts.
Steven Shepard has a profile of Ann Selzer in the Politico that you should read. Ms. Selzer does not take short cuts. Consequently, her polling is trusted by both parties and the media.
Some pollsters, like the CNN/ORC poll to which Trump referred, randomly call Iowa phone numbers and ask respondents if they’re registered to vote and whether they intend to go to a caucus site. Others, like Monmouth University, have used a tighter screen — only calling those voters who participated in recent primary elections. (Though Monmouth recently expanded its screen to include a minority of people who haven’t participated in past nominating contests.)
Selzer views the CNN poll as too broad, and the Monmouth method — which is shared by some political campaigns — as a methodological shortcut. Calling only or mostly frequent primary voters is too restrictive, Selzer warns, and compromises the sample in order to cut costs, since pollsters are calling only people who have demonstrated past voting behavior.
“Caucuses are a low-incidence event,” Selzer explained. “So if you start with a narrower universe, where the incidence is higher proportionately [among] people who’ve voted in Republican primaries before, your incidence is going to go up to 25, 30, maybe 40 percent. You’re hanging up on fewer people. It’s cheaper to do.”
Selzer, however, uses a list of all registered voters, but she lets those voters tell her whether they intend to caucus.
It’s a technique that she says has served her well in past cycles, even as some critics say the polls are including too many people who say they intend to caucus but never have before — groups that could favor candidates like Trump, and on the Democratic side, Vermont Sen. Bernie Sanders.