Determining the eventual winner of the Democratic primary contest, should one wish to do so, depends on tracking a number of factors. There’s national polling, giving a sense of who leads and who trails overall among Democratic voters. There’s polling in primary states, of course, though those polls tend to shift around a lot once voting starts.

Then there’s a less common bit of data: whom voters see as their second choice. Why is this important? Specifically because we can expect some shuffling once voting begins, because of shifted expectations about who will win and because of candidates dropping out of the race. In Iowa, second choices are critical, since the state’s caucuses force some voters to choose another candidate before the caucus is over.

On Tuesday, we got new data from Quinnipiac University looking at this question on a national level. In Quinnipiac’s poll, former vice president Joe Biden is the top choice of Democratic primary voters, followed by Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.). But Warren is the top second choice of respondents. Combine first and second picks, and Warren matches Sanders in support.

That by itself doesn’t mean a whole lot, since this isn’t how we calculate the winners of elections. It doesn’t mean a whole lot in this case particularly because so many of Warren’s supporters say Sanders is their second choice and vice versa. In other words, Warren may be the second choice of a lot of voters, but a lot of those voters have Sanders as their first choice, meaning that their second picks don’t do Warren much good if she’s trying to surpass him.

Mapping out Quinnipiac’s data on second choices, we see how the liberal-moderate split plays out. (Circles are scaled to overall support; line width is scaled to the percentage of a candidate’s support shifting to the targeted candidate.) Sanders and Warren see much of their support go to each other as a second pick. Former New York mayor Mike Bloomberg’s support goes largely to Biden and former South Bend, Ind., mayor Pete Buttigieg. Much of Sen. Amy Klobuchar’s (D-Minn.) support goes to Buttigieg as well.

(There’s a fascinating couple of Sanders supporters who identify Bloomberg as their second choice, perhaps confused about Sanders’s messaging on billionaires.)

When we compare support by demographic groups among the candidates, we see those same groupings. Sanders and Warren get more support from liberal primary voters; their opponents get more support from moderates, with Biden at the far end of that grouping.

The most dramatic split, though, is on age.

Sanders dominates among the youngest voters, while Biden does far better with older ones. In the abstract that’s interesting, but in practice it’s an advantage for Biden: Older voters are traditionally more likely to vote.

But back to the question at hand. There’s another factor that’s important to consider when looking at where support might shift: How solid is support for candidates in the first place?

Quinnipiac has an answer for that, too. Most Democratic voters say they might change their minds, shifting their support to another candidate. But those responses aren’t even: Most Sanders backers say their minds are made up, while most Warren supporters say they might shift. (Nearly all Klobuchar supporters say they could shift, which certainly isn’t great news for her.)

This, of course, continues to be the main story line of the primary. Things are unsettled, and support is soft. The arrival of voting next week may start to firm up voters’ opinions, but the available data makes it hard to determine where that firming may take place.

As is always the case, the best answer to the question of how the race will turn out is to wait and see how it turns out.