The marginal product of NBA players

Most of you have read Moneyball by now, so why not measure the marginal products of NBA players as well? After all, playoff time and the MVP award are just around the corner. Wayne Winston, professor of decision sciences at Indiana University, has tried to crack the numbers:

"Basketball's a team sport, and lots of things aren't tracked," Winston says. "Like taking the charge, going through a screen, tipping a ball to your teammate, saving a ball from going out of bounds. That's where our system comes in. All these little things should translate into points." One problem: Traditional plus-minus systems tend to overrate average players on good teams and underrate good players on lousy ones. After all, a zero plus-minus rating on the Los Angeles Lakers is not the same as a zero rating on the Los Angeles Clippers, mostly because one team has Kobe Bryant and Shaquille O'Neal and the other has Marko Jaric and Chris Kaman. To compensate, Winval's ratings are weighted to take into account every other player on the floor. For every time segment a player is in a game, the system tracks the other nine players on the floor, the length of the segment and the score at the start and end of the segment."

In other words, he tries to measure marginal product in terms of points, adjusting for the values of the other players. The system is called Winval, here is the article, the paper version has more information. And who comes out on top? Please sit down, the five best players in the NBA, according to this measure are:

1. Hedo Turkoglu (who? he plays for San Antonio but doesn't even start)
2. Vince Carter (a well-known star, but universally considered soft and a choker)
3. Kevin Garnett (the likely MVP for this year)
4. Brad Miller (very good player, but not elite)
5. Manu Ginobili (very good player, perhaps headed toward elite status)

Shaq, Kobe, and Tim Duncan are not in the top ten. None of the five, except for Garnett, crack a recent USA Today straw poll for NBA mvp. (By the way, here is last year's Winval list.) Now maybe these rankings are right and conventional wisdom is all wet. Marginal product is, well...marginal product. But what are some other options?

1. A player with a high rank could have a really bad replacement, thus boosting his measured marginal impact.
2. Some players are "momentum" players, they are put in when the tide is turning.
3. Some players are wonderful for the time they play but could not keep it up for the whole game. They look great when you see them, but they are not worth very much overall.
4. The econometric model is somehow misspecified, but of course you can always say this.

The bottom line: I'm still puzzled by how much the measurements diverge from common sense. The NBA offers gobs and gobs of measurable information. Yet intuition remains indispensable when we try to estimate marginal product. By the way, Winval predicts that Mitchell Butler (who?, 13.4 minutes per game) is the best Washington Wizard.