Even if you like numbers, you probably think the world needs a Next Great Stat like it needs another Miley Cyrus scandal. When it comes to advanced metrics, we've already got PAP and BABIP, VORP and WARP, wOBA and xFIP ... and that's just for baseball. Isn't enough enough?
No. We're here to tell you that an emerging statistic called Win Probability deserves your full attention. It's elegant, intuitive and revealing. And it will change the way you watch sports.
Win Probability is simply the expected chance that a team will win a game at a particular moment in time, given the situation it faces. A player's contribution to changing a team's odds from one play to the next is called Win Probability Added, or WPA. You remember that crazy Eagles-Giants game in Week 15? The Eagles, after trailing all day, not only climbed back from a 31-10 deficit to tie, but they also forced the Giants to punt from their own 29-yard line with 14 seconds left. At that moment, the Eagles had a 54 percent chance of winning. (We'll explain later how we know that.) Then Matt Dodge lined the ball to DeSean Jackson, who waltzed into the end zone as time expired, bumping Philly's Win Probability to 100 percent. The difference in the Eagles' Win Probability from before the punt to after it, 46 percentage points, represents the WPA of Jackson's return.
Now, just from this example, a few of Win Probability's cool features are clear. For one, we can use WPA to identify the most critical plays in any game. As it turns out, Jackson's punt return had by far the highest WPA of any play in that Eagles-Giants game. For another, in every contest, the players on both teams have a total of exactly one WPA to divvy up. That's because both teams start with a 50 percent chance of winning (or 0.5 wins), with one moving to 100 percent at game's end (one win) and the other to 0 percent (zero wins). In fact, WPA is actually measured in wins: Jackson's clinching TD was worth 0.46 wins to the Eagles. So by adding up a player's WPA over a season or career, we can get a handy snapshot of which players are truly most valuable. For example, through Week 15, Falcons QB Matt Ryan was leading the NFL with 4.43 WPA.
But the best thing about Win Probability is that it captures, like no other stat, how we viscerally experience sports. As fans, we carry in our guts a reflexive, fuzzy calculation of our team's chances of winning a game -- a nervous tension that explodes after triumphant plays and collapses after moments of agony. Win Probability expresses that emotion with mathematical precision. Throughout the fourth quarter of that Eagles-Giants contest, Win Probability showed what New York fans felt: An all-but sure thing (the Giants had a 99 percemt Win Probability when they took a 21-point lead with 8:17 left) decayed into what was essentially a 50-50 proposition, then to sudden disaster.
Of course, just because Win Probability is easy to understand doesn't mean it's a cinch to calculate. Its genesis dates back to 1969, when brothers Harlan and Eldon Mills leased an IBM 1620 mainframe and fed it punch cards to simulate and analyze baseball games. The Mills brothers were military pilots and talented data analysts. More important for the history of Western civilization, they were obsessed with trying to measure clutch hitting. Using their computer, they examined every event of the 1969 season, assigning win and loss points to the players involved in each of the season's 155,000 game situations. When they were done, the Millses published a book detailing their findings. Most notably, by their calculations, journeyman first baseman Mike Epstein of Washington was more valuable than the AL's actual MVP, Twins Hall of Famer Harmon Killebrew. Who knew?
Sadly, the book sold around 2,000 copies, not enough for the brothers to turn pro as sabermetricians. But as computers have become stronger and cheaper, a new generation of analysts has picked up the threads of their work. To calculate Win Probability today, some analysts run sophisticated simulations to see how often teams win under various sets of conditions. Others employ matrix algebra to solve baseball or football as giant series of equations. Either way, the results are endlessly fascinating. "I introduced my first WPA story the day after the Bartman/Cubs collapse game in 2003," says Tom Tango, co-author of The Book: Playing the Percentages in Baseball. "How can you capture that game with numbers? I think only WPA could do it. It's a story stat." In the eighth inning of that notorious playoff game against Florida, the Cubs' Win Probability crashed from 95.6 percent to 1.8 percent, according to Tango's calculations. But Steve Bartman's interference caused just 3.1 percentage points of the plunge.
In 2006, Tango started showing WPA in online graphs, which really communicate the emotional roller coaster of Win Probability. "The best games look like a heart attack on an EKG monitor," says Brian Burke of Advanced NFL Stats, who has pioneered the development of Win Probability in football. At Burke's site, you can now get Win Probability graphs for any NFL game since 2000, along with scores called "Excitement Index" (total WPA movement for both teams in one game) and "Comeback Factor" (the odds that a team would come back from its lowest point in Win Probability to win a game).
Fangraphs started tweeting Win Probabilities in real time in 2009. Last year, Baseball-Reference added WPA data to its huge trove of historical data, which are free and easy to use. Now, ESPN.com includes Win Probabilities in its GameCast play-by-play coverage of MLB games. You can even calculate WPA yourself for any game situation at The Hardball Times or Advanced NFL Stats.
Is there anything Win Probability can't do? Well, yes. If you're trying to figure out whether Joey Votto is going to outhit Prince Fielder next year or how much the Reds should pay their MVP, this is not your stat. WPA looks back, not ahead; it measures accomplishment, not skill.
But you know when you watch poker on TV, and the screen displays each player's chances of winning as the cards come out? And how you don't really need to understand anything about flops or implied odds to pick up the flow of a game -- you can just figure out what's happening as those little percentages change? Those numbers are Win Probabilities. And it's only a matter of time before they start popping up during baseball or football broadcasts. Unobtrusive? Check. Easy to comprehend? Check. Revealing? Check. Best of all, unlike most Next Great Stats, it's not just about brains. It's also about the heart.
Peter Keating is a senior writer for ESPN The Magazine.