Comments on: BBR Mailbag: More Statistical +/- Tidbits

By: Todd

Todd — Thu, 23 Apr 2009 20:03:58 +0000

Steals aren't the only way that a defensive player can force a turnover. Players can also force turnovers through offensive fouls or by tipping the ball out of bounds (as long as it touches an offensive player before it ends up out of bounds). Even when there is a steal, it is often credited to the player who ends up with the ball rather than the player who causes the turnover. That is why NBA teams often keep track of "deflections" instead of just steals and blocks. I would bet that the number of deflections a player gets correlates strongly with the number of steals they get, so the steals is probably acting as a proxy for deflections and for other defensive contributions that don't show up in the box score.

By: Ben

Ben — Wed, 22 Apr 2009 22:33:52 +0000

You're right Biggles. It's hard to see what the age^2 term is adding to the model.

By: biggles

biggles — Wed, 22 Apr 2009 22:07:54 +0000

Looks like adding the age^2 term doesn't make much difference unless you're Dikembe Mutombo. The quadratic for age peaks at 62, so the model still seems to say that as you get older, your value over and above your stats gets higher and higher (if only a bit).

By: NickS

NickS — Wed, 22 Apr 2009 19:46:48 +0000

No, because the regression coefficients are dependent upon the measurement scale of each predictor. So I was correct in my second comment? I know I've been dominating the thread somewhat, but would it be interesting to show a column that had each coefficient divided by the standard deviation for that factor?

By: Ben

Ben — Wed, 22 Apr 2009 19:44:36 +0000

Salary would be a fun variable too.

By: Justin Kubatko

Justin Kubatko — Wed, 22 Apr 2009 19:40:32 +0000

NickS wrote:

“Importance” was probably the wrong word there, but isn’t it fair to say that the high regression coefficient for steals implies that SPM places a high “value” on steals? (with “value” being deliberately vague there).

No, because the regression coefficients are dependent upon the measurement scale of each predictor. For example, steals occur much less often than assists, so it's not that surprising that the coefficient for steals is larger than the coefficient for assists.

By: NickS

NickS — Wed, 22 Apr 2009 19:33:13 +0000

The size of the regression coefficient does not really tell you anything meaningful about the relative importance of each predictor. "Importance" was probably the wrong word there, but isn't it fair to say that the high regression coefficient for steals implies that SPM places a high "value" on steals? (with "value" being deliberately vague there). I freely acknowledge that could be completely misreading the significance of that coefficient, and please correct me if I am.

By: NickS

NickS — Wed, 22 Apr 2009 19:29:11 +0000

it might be that it’s proxy for an unusual level of quickness . . . Sure. If steals are "overvalued" that would mean that they were showing up as a proxy for something else. The question then becomes whether there is some other factor that could be used along with steals to capture that "other quality" more precisely. Imagine, for example, that you added in a term for the number of times per 40min that a player scored an assist, a basket, or a shooting foul within 5 seconds of an opponent's TO or the team's defensive rebound and used that as a different proxy for speed and opportunism. If that worked then you would expect to see the coefficient for steals go down, as the new facts captures some of that value. The question then would be how much accuracy that added -- or whether it turned out that steals alone were a perfectly good measure of those other qualities. I am just trying to think this through out loud. I think that's all correct.

By: Justin Kubatko

Justin Kubatko — Wed, 22 Apr 2009 19:22:15 +0000

NickS wrote:

Is it just me, or does it seem odd that steals are, more or less, an order of magnitude more important than anything else.

The size of the regression coefficient does not really tell you anything meaningful about the relative importance of each predictor.

By: Ben

Ben — Wed, 22 Apr 2009 19:18:03 +0000

I believe steals show up as significant for offense as well as defense. It could be simply that they lead to high % fast break opportunities, but it might be that it's proxy for an unusual level of quickness, basketball IQ, and hand eye coordination that is helping in all kinds of plays not tracked by traditional stats.