I'm not that familiar with all the studies that have been done to validate or critique WP.
But I just took a quick look at how its calculated (
here), and something stuck out that's bothering me.
They develop a model for Win% based on team offensive efficiency and defensive efficiency. They use the standard definition for offensive efficiency, but they define defensive efficiency as "points surrendered divided by possessions acquired (PA)", where:
Code:
PA = DFGM + 0.45*DFTM + REBD + DTO + REBTM
Despite the very different formulation, their defensive efficiency yields similar rankings to the more commonly used definition of points surrendered divided by opponent possessions.
Anyway, so they break up offensive efficiency and defensive efficiency into 4 terms: points scored, possessions employed, points allowed, possessions acquired and calculate the marginal value for each factor. For PA, its a 0.033, meaning every possession acquired is equivalent to .033 wins. For points surrendered, its -0.032. So, a made 2-point field goal by the opponent (2 points surrendered, 1 possession acquired) would effectively cost a team .031 wins.
A defensive rebound, by their formula, is (I would think) equivalent to an acquired possession and therefore should add .033 wins. In their table, they have it add .034 wins -- not sure why.
An offensive rebound would be equivalent to subtracting a "possession employed (PE)", and therefore should add .034 wins, which is what they show.
But how you define the formula for "PA" or "PE" changes the value of the individual statistics. So, take PE:
Code:
PE = FGA + .45*FTA + TOV - REBO
One could have defined it in this way instead:
Code:
PE = FGA_not_offensive_rebounded + .45*FTA + TOV
The difference is that REBO no longer shows up in the formula. Now, if a player attempts a shot that is offensive rebounded, it does not contribute at all to PE. So, the player isn't penalized and the offensive rebounder isn't credited. Now, I wouldn't say that's fair, but then I don't think its fair to say that a missed field goal is equivalent to a turnover and an offensive rebound is equivalent to stealing it back. And that's effectively what the WP model says simply because of the way they formulate PE/PA, which is, I think, arbitrary on their part.