Bucks Diary

Monday, November 27, 2006

Freakonomic error in the Eff48 statistic


Since I started this blog I have argued (and its hardly an original argument) that raw statistics such as scoring average and rebounding average are wildly overvalued in basketball. They present an incomplete and often misleading indication of a player’s value to his team. Thus I ignore them completely. A player’s value is better understood by considering the entirety of his statistical contributions, crediting him for positive production while penalizing him for negative production, and then converting the result into something that is playing-time neutral. The NBA’s efficiency statistic (Eff48) does all of that, and until now I thought it was a bulletproof measure of productive value. I wasn’t quite correct.

In the book “The Wages of Wins” (a book aptly described as “Freakonomics meets ESPN”), two economists point out the flaw in my thinking. While they agree that traditional statistics provide a highly distorted reflection of productive reality, they disagree that Eff48 represents a full correction.

Their critique is simple – Eff48 doesn’t adequately penalize missed field goals. Under Eff48 every missed field goal costs just one efficiency point, while every made field goal adds at least 2. Thus the formula indirectly rewards shot attempts over shot efficiency.

What’s missing in Eff48 is an acknowledgement of the opportunity cost of missing field goals. A player who takes 15 two-point shots and makes just 5 of them has theoretically cost his team at least twice as many potential points (20) as he has delivered in actual points (10). Therefore the true productive value of his field goal attempts should be somewhere in the red, yet under Eff48 he breaks even.

That said, Eff48 is still the best, most understandable, and most accessible measure of a player’s true productive value, and it will be the statistic I will continue to emphasize. (In plain English that translates into “I’m not good at math, so if you think I’m going to waste my time doing my own calculations based on a complicated formula I don’t really understand, you’re delusional.”)

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