Saturday, February 1, 2014

New Stat: "Kitchen Sink" Win Probability Added

I have added a new column to my player win probability page. For lack of a better term, I am calling it Kitchen Sink WPA (as in "everything but"). It quantifies the win probability contributions for every box score stat we can measure and attribute at the player level. That means it includes all the stats in my "official" WPA stat: shots (made and missed), getting to the foul line, free throws (made and missed), and turnovers. In addition, it includes Win Probability Added due to rebounds, assists, blocks, and steals.

This new stat will not replace my official definition of Win Probability Added, it's just an alternate way of looking at how a player has contributed to team wins. I consider this an unofficial stat because I think I'm adding apples and oranges here (or orange juice and toothpaste). Official WPA captures both the upside and downside of a particular subset of player contributions. It measures what a player does when entrusted with his team's possession.  WPA due to rebounds, assists, blocks, and steals are "plus only" stats. They can only help a player, which is why I have chosen not to include them in official WPA.

Here is the player win probability table sorted by "kitchen sink" WPA.

I have abbreviated the stat as "kWPA" in the table. As of February 1, Stephen Curry leads the league in kWPA. He was already number 13 in official WPA, and his 11.97 in assist WPA vaults him to the top of the rankings. In general, kWPA will be much kinder to point guards than official WPA. WPA, perhaps unfairly, counts turnovers but not assists. Point guards tend to have more turnovers not because they are bad ball handlers, but because they are usually tasked with running the offense and distributing the ball to their teammates.

Lebron James, a fixture at the top of the WPA rankings, also does well by the kWPA metric (#2, behind Curry). His chief competitor for WPA, Kevin Durant, is worse off, as he does not amass the kind of box score stats (particularly assists and rebounds) that Lebron does.

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