There are a lot of reasons why no one can say with certainty what will happen in Iowa. Polling on the caucuses is hit-or-miss, given the nature of the thing. Lots of people won't decide until Monday whom to vote for.

The real question mark, as we've noted repeatedly, is how many people come out to caucus.

To that end, several outlets have reported on Ted Cruz's "magic number" in Iowa -- i.e. the level of turnout that makes or breaks his chances of winning the state. The National Review puts it at 135,000. Politico has a range from 135,000 to 150,000. The idea being that it's a simple yardstick against which you can make predictions about who will win. Turnout's under that number? Cruz wins. It's over? Donald Trump does.

Notice that 135,000 to 150,000 is a big range, a swing of 10 percentage points. So what's really going on here?

In the past two Republican caucuses, turnout has been at about 20 percent. 2012 set a new record for the number of Republicans who came out to caucus, but it was actually a slightly lower percentage because the population of Republicans had grown.

If turnout is at 135,000 today, that's a turnout of about 22 percent, based on current registration.

The reason this could matter is simple: Trump does much better with first-time caucus-goers, as the Des Moines Register/Bloomberg poll from Selzer & Co. that came out this weekend makes clear.

Trump leads among all voters by 5 points. Among those who would be caucusing for the first time, he leads by 16.

So the magic number, in other words, is "where we'd expect turnout to be if the regular people turn out." Anything at or below that level -- which is what Selzer predicts -- and Cruz wins. Why? Because among people who have caucused before, Cruz leads by 3 points.

In other words, if turnout is normal -- or even slightly higher -- Cruz wins. If it's a lot higher than normal? Trump does.

Probably, anyway. Lots of things can and have happened. The magic number to watch? The one they announce once the caucus is over which says what percentages each candidate received.