by Sam Wang
Co-founder, Princeton Election Consortium.

I began what is now the Princeton Election Consortium (PEC) by analyzing the 2004 Presidential race, which came down to Ohio, Pennsylvania, and Florida. The 2014 Senate race is just as suspenseful, and comes down to a handful of states that includes Alaska, Arkansas, Colorado, Iowa, and Louisiana. So before I start, let's keep in mind that the true stars of this story are not statistical analysts, but the candidates and the voters - and control of the United States Senate.

Recently a well-known commentator, Nate Silver, has taken to criticizing the Princeton Election Consortium approach. However, he has made a number of factual and conceptual errors. If an experienced analyst like him could make those misreadings, so could many people. PEC has a track record equal to (and for the Senate, superior to) commercial outlets such as FiveThirtyEight. In addition to correcting the errors, I will explain how we do it. By "we" I include my readers: PEC's code has always been free to all, and my commenters compose a small army of experts. In fact, it was their vigorous factchecking of recently published criticism of PEC that encouraged me to respond.

1. The PEC prediction sits at the end of a very narrow range.

The truth is this: neither Nate Silver nor I know which way the Senate will go. Indeed, in the current situation, a few thousand students in Ames, Iowa and Boulder, Colorado could help determine control of the United States Senate by getting out the vote in their states. As a fan of elections and democracy, I find moments like this to be amazing. In all the major models I see plenty of uncertainty. The Princeton Election Consortium's long-term estimate has fluctuated between 50% and 70% probability for Democrats and Independents holding 50 or more seats (and never 90% as implied by Silver). This is not much better than a coin toss. This is confirmed if one goes to The Upshot's "make your own forecast" interactive tool and selects "polls only" and unchecks "house effects" to get a result of 51% for Democratic control.

Accurate predictions in this range can go to the perceived underdog one-third to one-half of the time. Many people make the conceptual error of mentally rounding up to 100% any probability that is even a hair over 50%. For this reason, probability may not be a good way to quantify the current situation. I urge all readers to look beyond probabilities to see an extremely close Senate campaign, in which each side could well end up with 49, 50, or 51 seats. So the question "is the probability above or below 50%?" is excessively simpleminded.

At PEC we prefer to use measures such as the Meta-Margin, defined by how much opinion, as measured by polls, would have to swing to create a perfect tie. It can also be used to estimate how much collective pollster error would be required in order to change the estimate of likely Senate control. With a current Meta-Margin of R+0.9%, this is anybody's game.

2. How does the PEC approach eliminate pollster bias?

Of perhaps greatest interest is the fact that on Election Eve in 2012, PEC called every close Senate race correctly - 10 out of 10. Silver is protesting against a model that has consistently matched or outperformed his own calls since he came onto the scene (see 2008, 2010, and 2012). PEC does this in a different way than the FiveThirtyEight method of detailed analysis of pollster "house effects" and "fundamentals." The FiveThirtyEight approach can work in the right hands, but is laborious and mostly converges with a simpler polls-only approach in the closing stages of a campaign.

A key to PEC's success is our use of medians as a statistical tool. Other sites use averaging, which tends to overly weight outlier polls. PEC's use of medians reduces the influence of outliers without having to dissect individual pollster performance. Only one recent poll from each pollster is allowed into the daily "snapshot" of current conditions. More generally, the PEC philosophy is to provide a prediction model based entirely on the history of all available likely-voter polls as found in the HuffPollster database. (Oddly, Silver claims that we use a lot of registered-voter polls, which we don't.)

more....then, could someone explain this in simpler terms?