I’ve begun using some new stats here lately, and I want to take the opportunity to explain them clearly. Readership has been increasing rapidly too, and some readers aren't as familiar with the terms we use around here. This post will serve as a reference each time I use the new stats in the future.
Although it’s been used in baseball sabermetrics for several years now, Win Probability Added (WPA) is a new, or at least rediscovered, concept for football stats. It measures each play in terms of how much it increased or decreased a team’s chances of winning the game.
WPA starts with a Win Probability (WP) model of the game of football. Every situation in a game gives each opponent a particular chance of winning, and a WP model estimates those chances. The model created here at Advanced NFL Stats uses score, time, down, distance, and field position to estimate how likely each team will go on to win the game. For example, at the start of the 2nd quarter, a team down by 7 points with a 2nd down and 5 from their own 25 will win about 36% of the time--in other words a 0.36 WP.
On that 2nd down and 5, let’s say there is a 30-yard pass, setting up a 1st down and 10 on the opponent’s 45. Now that team has gone from a 0.36 to a 0.39 WP. The WPA for that play would be +0.03.
If instead the quarterback throws an interception returned back to the line of scrimmage, the opponent now has the ball at the 25, giving the trailing team a 0.28 WP. The WPA for the interception would be -0.08.
WPA is very sensitive to the context of the game. That same interception that cost -0.08 when a team was down by 7 points in the 2nd quarter would cost much more if it the offense was leading by a point late in the 4th quarter. Putting your opponent in immediate field goal range would be nearly fatal.
Stats are tools, and each tool has its own purpose. WPA is what I call a narrative stat. Its purpose is not to be predictive of future play or to measure the true ability of a player or team. It simply measures the impact of each play toward winning and losing.
WPA has a number of applications. For starters, we can tell which plays were truly critical in each game. From a fan’s perspective, we can call a play the ‘play of the week’ or the ‘play of the year.’ And although we still can't separate an individual player's performance from that of his teammates', we add up the total WPA for plays in which individual players took part. This can help us see who really made the difference when it matters most. It can help tell us who is, or at least appears to be, “clutch.” It can also help inform us who really deserves the player of the week award, the selection to the Pro Bowl, or even induction into the Hall of Fame.
As interesting as that might be to us as fans, WPA might be even more useful to coaches and strategists. We can measure whether a kicker’s high field goal accuracy was worth a trade-off in short kickoffs. And for the first time, we can measure the effectiveness of clock management strategies—is it better to run three times and punt, or pass and go for the first down to end the game?
Even better, WPA can measure the risk-reward balance of a given situation. For example, when is it best to take a sack, and when is it best to take your chances throwing into traffic? Or for defenses, when is it best to roll the dice on a big blitz and risk a long completion due to a big hole in the secondary? We can measure which interceptions were the most costly, and which quarterback tends to throw the costliest ones. WPA may actually be able to measure decision-making ability on the field and on the sideline.
WPA is not ‘the one stat to rule them all,’ and it does have its disadvantages. It can certainly be improved in several respects, and I’m already working on several ideas. As time goes on, we’ll find even more new and interesting applications for it, and this is only the beginning. More than any other sport, football is about strategy, risk, and reward, and WPA is well suited to capturing much of what makes football so compelling.