Just over three months before Iowans caucus for Democratic presidential candidates, four polls out this week present a muddled picture of a primary race that is unfolding in varying, and sometimes conflicting, ways nationally and in early-voting states.

A pair of new national polls present starkly different results. A CNN survey released Wednesday had former Vice President Joe Biden with a commanding lead of 15 percentage points ahead of Sen. Elizabeth Warren of Massachusetts. But a Quinnipiac University poll released Thursday had Warren as the front-runner, 7 points ahead of Biden.

Sen. Bernie Sanders of Vermont finished a strong third in both national polls, suggesting that his standing in the race has not been diminished by his heart attack this month — and that he may have been buoyed by endorsements from Reps. Alexandria Ocasio-Cortez of New York and Ilhan Omar of Minnesota.

Two other polls from early-voting states show a tight race in Iowa and Biden maintaining a comfortable lead in South Carolina.

Pete Buttigieg, the mayor of South Bend, Ind., continued to show particular strength in Iowa, earning third place and 13% support in the Suffolk University/USA Today poll of Iowa Democrats released Monday. Sen. Kamala Harris of California, who is now staking her campaign on a top-three finish there, was in a four-way tie for fifth in that poll with 3% support.

It is not clear why the CNN and Quinnipiac polls present such divergent results for Warren and Biden. But the polls, both considered to be of high quality and able to help candidates qualify for the next Democratic debate, add data points to a divide that has emerged in recent weeks: between surveys that show Biden with a commanding lead and those that have him in a statistical dead heat with Warren.

A number of things can affect a poll’s results, including the wording of a horse-race question and the order that items are asked. In CNN and Fox News polls this year, respondents have typically been asked for their opinions on each of the Democratic presidential candidates, among other questions, before being queried about their vote preference.

In Quinnipiac’s polls — as well as those conducted by Monmouth University, which have also shown Warren climbing steadily — people have not typically been asked to evaluate the candidates one-by-one before giving their vote choice.

If such small differences in survey structure are indeed having an effect on results, it may reflect the fact that many respondents are not yet certain about their feelings.

The CNN poll released this week brought that point home, finding that a majority of Democratic voters who favored one candidate — 53% — said they could still change their mind about whom to support for the nomination.

“It makes me think that voters are not settled, that they’re still shopping for a candidate — that’s why you’re seeing some movement between polls,” Doug Schwartz, Quinnipiac’s polling director, said Thursday.