What can we learn from national primary polling? Virtually nothing: who's up? who's down? who cares?

• Author: Slade, Stephanie

In the spring and early summer of 2007, the frontrunners for the Democratic and Republican nominations for president were--by healthy double-digit margins--New York Sen. Hillary Clinton and former New York City mayor Rudy Giuliani. Oops.

It's become a cliche to joke about these comically inaccurate results from exactly eight years ago, but they are far from the exception. National survey research, even right before primary elections and caucuses, is almost useless as a barometer of who is most likely to win a party's nomination-except in those circumstances when one candidate is so far ahead that no polling is necessary to know it.

This fact will not deter the press from spilling barrels of real and virtual ink writing about each new data point for the next 12 months. "A new poll of possible 2016 presidential contenders shows a crowded Republican field," begins a recent, and utterly typical, article from the International Business Times, "while former Secretary of State Hillary Clinton has a wide lead on the Democratic side."

But primary polling does not measure who is most likely to win a party's nomination. Betting markets, such as the now-defunct Intrade, which shuttered its operations after federal regulators brought suit against it in late 2012, come closer to doing that. They ask participants to look into the future and guess what public opinion will be like on the actual day of the election. Primary polling, on the other hand, measures whom people would support were the voting to happen right now.

The further out from Election Day a survey is conducted, the less likely the result is to hold. National polls taken months before the first primaries are often astonishingly bad at forecasting how the chips will have fallen by the end of a several-months-long series of nominating contests.

Consider the 2008 race. Summer 2007 saw Hillary Clinton leading all comers by an intimidating margin. National surveys conducted for ABC News and The Washington Post in June and July of that year put her 15 percentage points ahead of her main opponent, Illinois Sen. Barack Obama. In December, mere weeks before the Iowa caucuses, an NBC News/Wall Street Journal poll found her winning by 22 percentage points. Even heading into Super Tuesday (February 5), most national polls gave the former First Lady an edge. Nonetheless, Obama became the nominee.

Things were even more volatile on the Republican side of the aisle. Through the summer of 2007, Giuliani ran...