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Just How Arbitrary Is Fox's 10-Person GOP Debate Cutoff?

All five of these people are running for president, but it looks like only one will make it into the first Republican debate.

Welcome to the most exciting fight for tenth place you've ever seen.

It also just might be a meaningless fight.

With the major contenders for the GOP nomination now numbering 17, Fox News will only allow the top 10 candidates into the first GOP debate on Thursday. To determine the participants, Fox will be averaging together five national polls.

And that methodology — what Fox has said about it as of right now, that is — has political scientists up in arms. The Marist Institute of Public Opinion suspended polling this week because it was worried polls were being used to make too fine of distinctions between candidates.

Averaging polls together can diminish the margin of error to a degree, but not enough to make these candidates easily rankable, as the Fox cutoff demands.

I took a stab at averaging the latest five polls together and determining the resulting margin of error for each candidate with the guidance of Samuel Wang, neuroscientist and founder of the Princeton Election Consortium blog. Here's a look at the results.

The latest five national polls (as of midday Tuesday) had maximum margins of error of plus or minus 4 to 6.2 percentage points on questions about which GOP candidates they support. Averaging them together (see my spreadsheet of step-by-step math here) clearly diminishes that for most candidates — but not enough to (in this example) reduce the overlap between 10th-place Kasich and 11th-place Perry.

So if Kasich makes it in, Perry could make a good argument that the difference between his support and Kasich's support isn't meaningful enough to keep him out of the debate. And then if Perry gets in, look at Rick Santorum, who also overlaps with Perry — perhaps he should be in. In which case maybe Jindal should. And so on.

To be clear, this is just an example of how margins of error could look. Fox could compute it another way, and they could still potentially decide to use different polls. (The network has said it will only use national polls from nonpartisan firms that use live callers.)

Or, instead of simply averaging candidates' support levels together (as I did above), they could choose to weight polls by their sample sizes — Twitter user @Taniel has been tracking the differences between the two different averaging approaches in a helpful Google spreadsheet.

Also, consider that the polls didn't all measure the same populations — Fox, for example, surveyed likely Republican primary voters about their nominee preferences. Bloomberg, meanwhile, spoke to "Republicans or Republican leaners."

There may be a million ways you can nitpick how and why Fox will do its debate math (and you'll hear a lot of it during the 48 hours between Fox's debate field announcement Tuesday evening and the debates themselves on Thursday night).

Still — however you do the math, using polls to cut down the debate field in this way risks stopping some candidates' runs by arbitrarily cutting them out of the conversation before the campaign has really even begun. (Fox is hosting an earlier debate for the lower-ranked candidates — a debate that many have likened to a "kids' table.")

"Polls are not very useful right now except for telling you about tiers of candidates. They really tell you that Trump has more support than Christie and Rand Paul. It really tells you about tier of public visibility," said Cliff Zukin, professor of public policy at Rutgers University and former president of the American Association of Public Opinion Researchers.

It's true that polls have been used to limit debate fields in the past — Fox spokeswoman Carly Shanahan pointed out to NPR that it in the past Fox has used polling to determine debate participants — in a 2011 South Carolina debate, the network limited participation to anyone polling at 1 percent or above.

Still, a 1-percent cutoff is designed to include everyone with legitimate (if small) support. The top-10 cutoff this year is designed to winnow down the field. The idea this time around appears to be exclusivity, rather than inclusivity.

Think of it in terms of the tiers Zukin talks about: cutting the field between 10 and 11 seems to cut the bottom tier in an arbitrary place. Arguably, Kasich and Santorum are in the same "tier." But one will likely make it in while the other won't.

"I suppose Fox hoped that a top tier would emerge by the time the first debate rolled around. But based on current polling, there's no good rationale for arbitrarily selecting a top ten," said Patrick Murray, director of the Monmouth Polling Institute, in a press release about his group's latest poll.

Fox spokeswoman Shanahan didn't respond to questions about Fox's methodology but has directed NPR to statements made by Fox in other outlets on Fox's criteria.

"National polls are the traditional, time-tested yardstick by which presidential hopefuls have long been measured and remain the fairest, most objective and most straight-forward metric for gauging the viability of these candidates," Michael Clemente, executive vice president of news for Fox, told Bloomberg.

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