THE BOOK--Playing The Percentages In Baseball

Thursday, January 28, 2010

WPA explained - football style

Brian explains WPA, from a football perspective.  I agree with him that the value of WPA is about the in-game strategy:

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.’… 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.

I will disagree with him somewhat here:

We can also add up the total WPA for individual players to see who really made the difference on a team. It can 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.

In point of fact, adding up the WPA for individual players does not tell you who “really” made the difference on a team.  It does tell you who was involved in the most impactful and least impactful plays.  Now, it’s a given that there is some correlation between talent and finding yourself in impactful plays: the best and worst players are involved in a disproportionate number of times in the most impactful plays.  So that, after many many many such plays, the noise of the stat gives way to the signal.  But, I think it’s best to stay away from the kind of association to sell the stat. 

And he’s got it right here:

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.

WPA is about capturing the story in a quantitative fashion, without necessarily ascribing it to the talent of the player.  Rather the association should simply be about who was involved when great and bad things happened. Mark Prior was involved in alot of bad things that fateful day in 2003 against the Marlins.  It doesn’t necessarily tell us anything about Prior himself.

In addition, WPA, or really, WE (win expectancy) tells you about the strategy implications.