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Ed Küpfer
Joined: 30 Dec 2004
Posts: 787
Location: Toronto
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 Posted: Mon Jan 17, 2005 12:15 am Post subject: 4 Factors, times two |
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I gotta tell you, I really like the four factors idea. At the team level, it's a great compromise between the simplicity of ORTGs and DRTGs, and pouring over the multitudes of stats that track every single thing that happens on the floor.
(Don't know what I'm talking about? Dean Oliver discusses the concept of four factors here, and I wrote a little something on it too.)
Dean mentions his weights in his article -- shooting percentage (10), turnovers (5-6), offensive rebounding (4-5), getting to the foul line (2-3) -- derived from his Roboscout program. I wanted to see if I could duplicate them.
Taking every team from 1974 to 2004, I regressed the 8 factors (4 on offense, 4 on defense) against win%. Specifically, the factors I used were standardized (by season) EFG%, OR%, TO%, and FOUL% -- the last two being Turnovers and Fouls per possession (Foul = .44 * FTA), and I forced the intercept to equal zero. Here's how it looks:
| Code: | Var Coef
O_EFF 10.0
O_TO -5.9
O_OR 4.1
O_FOUL 2.9
D_EFF -8.7
D_TO 5.6
D_OR 3.5
D_FOUL -3.9 |
The correlation between predicted and actual win% was 0.94, with everything being significant at 0.1%.
Pretty close to what Dean got, but I wonder why defensive shooting was relatively less important than offensive shooting?
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Dan Rosenbaum
Joined: 03 Jan 2005
Posts: 541
Location: Greensboro, North Carolina
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| Posted: Mon Jan 17, 2005 2:27 am Post subject: |
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Nice work again, Ed. But if we included true field goal percentage (rather than effective field goal percentage), i.e. if we included free throws in the field goal percentage, I bet we could knock it down to three factors: shooting, rebounding, and turnovers.
But then we would want to know what generates good true field goal percentage. My bet is that this is where assists would come into play. They would help explain why some teams have better true field goal percentage than others. It doesn't make sense to include assists in the original regression, because holding field goal percentage constant, it is hard to imagine why an assist would have any value. Its value is almost entirely subsumed in the field goal percentage.
I think sometimes this "four factors" analysis has resulted in folks devaluing assists. And I think that is a big mistake. Because assists, especially assists from big men or high volume shooters, are highly correlated in my work with being a player that helps a team win.
Point guard assists, especially from point guards that don't score a lot, are much harder to value. It appears that a lot of these assists might be "system assists," which make valuing point guards by far the most difficult position to value using box score statistics. |
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benji
Joined: 31 Dec 2004
Posts: 32
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| Posted: Mon Jan 17, 2005 12:30 pm Post subject: Re: 4 Factors, times two |
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| Maybe looking at the difference between offense and defense would be better than each individually? |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 787
Location: Toronto
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| Posted: Mon Jan 17, 2005 2:31 pm Post subject: Re: 4 Factors, times two |
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| benji wrote: | | Maybe looking at the difference between offense and defense would be better than each individually? |
You're right: Offensive and Defensive Ratings predict win/loss percentages better than the four*2 factors do. (Why? I don't know. Is there some information contained within RTGs nor contained in 4 Factors? I don't think so...) The problem is that while looking at RTGs can accurately tell you how good a team is, it can't tell you why the team is good. That is, RTGs can't specify which things a team does well.
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Kevin Pelton
Site Admin
Joined: 30 Dec 2004
Posts: 979
Location: Seattle
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| Posted: Mon Jan 17, 2005 8:55 pm Post subject: Re: 4 Factors, times two |
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| Ed Küpfer wrote: | | Pretty close to what Dean got, but I wonder why defensive shooting was relatively less important than offensive shooting? |
I don't think that really surprises me. Most seasons, offensive rating tends to be a somewhat stronger indicator of team success than defensive rating. The implication I've drawn from that and the fact that there is typically more variance in offensive ratings than defensive rating is that offense tends to dictate how a matchup between two teams plays out slightly more than defense.
What's interesting to me is that fouls show as much more important on the defensive end of the court, implying that avoiding fouls is more of a skill than drawing them is.
As far as the loss in predictive value by going from two factors to eight, I'd assume that's because the relative value of each factor is not constant from team to team. Rebounding is assumedly more important for a team that's defensive-minded than an offensive one, for example. |
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benji
Joined: 31 Dec 2004
Posts: 32
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| Posted: Mon Jan 17, 2005 11:03 pm Post subject: Re: 4 Factors, times two |
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| Ed Küpfer wrote: | | You're right: Offensive and Defensive Ratings predict win/loss percentages better than the four*2 factors do. (Why? I don't know. Is there some information contained within RTGs nor contained in 4 Factors? I don't think so...) |
Actually, I meant, since you split the four factors into offense and defense...maybe finding the difference in each of the four would work better...I probably wasn't (and still aren't, heh) clear enough...
As for the ratings predicting it better...I like the Wizards as an example:
http://members.rogers.com/edkupfer/nba/8factors2_files/wasgraph.htm
They and their opponents are both below average at eFG%, and equal on to and foul. But their rebounding is so good it seems makes up for it all. Of you don't subtract offensive rebounds (or add them even) their points per 100 difference from their opponents becomes nearly nonexistant and their offense doesn't look so great...but when you're counting them their offense is one of the best in the league. In this case it appears (to me at least) that offensive rebounding is more important to them than the average team to make up for their low shooting.
The weights on average might be the ones noted, but it may be different for each team and that's why the offense and defense ratings predict the win% better. |
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HoopStudies
Joined: 30 Dec 2004
Posts: 706
Location: Near Philadelphia, PA
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| Posted: Tue Jan 18, 2005 12:16 pm Post subject: |
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| Dan Rosenbaum wrote: | It doesn't make sense to include assists in the original regression, because holding field goal percentage constant, it is hard to imagine why an assist would have any value. Its value is almost entirely subsumed in the field goal percentage.
I think sometimes this "four factors" analysis has resulted in folks devaluing assists. And I think that is a big mistake. Because assists, especially assists from big men or high volume shooters, are highly correlated in my work with being a player that helps a team win.
Point guard assists, especially from point guards that don't score a lot, are much harder to value. It appears that a lot of these assists might be "system assists," which make valuing point guards by far the most difficult position to value using box score statistics. |
Dan says this well.
Assists in some perspectives are valueless, but they clearly have value. At the team level, assists add nothing to explaining winning that isn't in efffg%. But efffg% and assists are related. I don't want to say one causes the other even though that is a tacit assumption of assists. At the large scale, you have "system assists" as Dan says that are a result of just having guys at the point slot (there are "system assists" at pretty much every position, but PG has most).
Guys who get double-teamed for their scoring -- if they have good assist totals, it is huge. Dan's work shows it. My work shows it. It's theoretically sensible. Typically these are big men, but it can be other guys. MJ was a good example of a non-post player who had to be double-teamed. Kobe gets good value for his assists. There are others but not a lot.
I don't think this is a surprise if you step back from the basic four factor analysis. But, as Dan said, some have devalued assists. I remember that even Dan initially undervalued them (a long time ago now) in one sense by implementing my indiv offensive rating formula with very little (or no?) weight on assists.
As to why the four factors don't predict win/loss as well, it's simple. The four factors go towards explaining ratings. Fundamentally, ratings determine winning. If you have some regression to try to explain ratings, you can't explain those at 100%, so you will do worse in explaining winning.
And, yeah, I think four factor analysis, in its basic simplicity, is one of the best things I've done. So incredibly obvious and powerful that it's amazing that it took so long to just come out and say it.
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Dean Oliver
Author, Basketball on Paper
The postings are my own & don't necess represent positions, strategies or opinions of employers. |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 787
Location: Toronto
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| Posted: Tue Jan 18, 2005 1:17 pm Post subject: |
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WRT assists, I will repeat my position.
1. My little study of 30 or so games showed that when a player shot in a potential assist situation, there was about a 20% increase in FG%.
2. I had a lot of trouble reconciling my assist scoresheets with official boxscores. Either the official scorers used a much looser definition of assist than I can even conceive, or they were padding the numbers.
3. Subsequently, I had a conversation with a guy employed by one of the NBA stadiums to run the stats displays on the stadium screens. He told me that he felt the numbers were innaccurate due to the poor ability of the official scorers, many of whom received their jobs due to nepotism.
The last point is hardly evidence of anything, but it served to reinforce my feeling of unease with the official assist numbers. As I result, I am using the old fashioned solution to problems of every kind: I am ignoring it, and pretending no problem exists.
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Dan Rosenbaum
Joined: 03 Jan 2005
Posts: 541
Location: Greensboro, North Carolina
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| Posted: Tue Jan 18, 2005 2:05 pm Post subject: |
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| Ed Küpfer wrote: | WRT assists, I will repeat my position.
1. My little study of 30 or so games showed that when a player shot in a potential assist situation, there was about a 20% increase in FG%.
2. I had a lot of trouble reconciling my assist scoresheets with official boxscores. Either the official scorers used a much looser definition of assist than I can even conceive, or they were padding the numbers.
3. Subsequently, I had a conversation with a guy employed by one of the NBA stadiums to run the stats displays on the stadium screens. He told me that he felt the numbers were innaccurate due to the poor ability of the official scorers, many of whom received their jobs due to nepotism.
The last point is hardly evidence of anything, but it served to reinforce my feeling of unease with the official assist numbers. As I result, I am using the old fashioned solution to problems of every kind: I am ignoring it, and pretending no problem exists. |
I am not so worried about the errors in these assist stats. It is a problem, but given the whole host of problems we have to deal with, it seems like a small one to me.
I have tried doing a little of this charting myself recently, and I noticed a few problems.
1. What did you do on breakaway fastbreaks, where there is no possibility of an assist (except in those cases where an assist faciliatates a more exciting dunk)? If we are trying to measure the benefit of an assist, it seems that these attempts should be ignored.
2. What did you do with offensive rebounds that almost immediately led to shots? There usually is no chance of assist on these field goal attempts, so it seems that they also should not be counted.
3. What did you do with foul shots that did not come as the result of a shot attempt? In this case it is unclear whether to count these attempts as assisted or unassisted, since there is no field goal attempt.
Trying to count these cases, I suspect, would result in unassisted field goals generating more points. So if this is what you did, I suspect that the "assist" benefit that you measured might be understated. |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 787
Location: Toronto
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| Posted: Tue Jan 18, 2005 2:19 pm Post subject: |
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| Dan Rosenbaum wrote: | | 1. What did you do on breakaway fastbreaks, where there is no possibility of an assist (except in those cases where an assist faciliatates a more exciting dunk)? |
I was being as generous as possible: I almost always gave an assist to a passer on a fast break, unless the break came on a steal and the stealer took the other way on his own.
| Dan Rosenbaum wrote: | | 2. What did you do with offensive rebounds that almost immediately led to shots? |
These maybe should've been counted separately.
| Dan Rosenbaum wrote: | | 3. What did you do with foul shots that did not come as the result of a shot attempt? |
I did not give an assist. The logic being, the player did something on his own to get fouled, and he should get as much credit as a player getting fouled on an iso play.
I'm not wedded to this, though. I can see tracking these separately.
| Dan Rosenbaum wrote: | | Trying to count these cases, I suspect, would result in unassisted field goals generating more points. So if this is what you did, I suspect that the "assist" benefit that you measured might be understated. |
Yes. I was being as conservative as possible -- I simply couldn't believe how off my assist numbers were. I'd like to know if the errors are random or systemic.
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kjb
Joined: 03 Jan 2005
Posts: 865
Location: Washington, DC
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| Posted: Tue Jan 18, 2005 5:30 pm Post subject: |
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| Ed: I'm seeing something similar as I'm tracking defensive stats for the Wizards. I'm tracking both blocks and steals, even though I'll use the official numbers when I'm putting together my defensive box scores. But I am noticing significant discrepancies between what I record and what the official box says. Specifically, I credit more blocks and fewer steals than does the official scorekeeper. |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 787
Location: Toronto
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| Posted: Tue Jan 18, 2005 5:52 pm Post subject: |
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| WizardsKev wrote: | | I credit more blocks and fewer steals than does the official scorekeeper. |
So it wasn't just me -- I am happy to hear that  Can you estimate by how much the numbers are off? My assist counts were off by between 5-10%.
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kjb
Joined: 03 Jan 2005
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Location: Washington, DC
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| Posted: Tue Jan 18, 2005 6:56 pm Post subject: |
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| I'll look through my scoresheets and see. I do remember one game in particular where Hughes was credited with 6 steals while I had him with 4, and Haywood was credited with 3 blocks when I had him with 5. The next game, Haywood officially had 1 block, when I had him at 3. Steals I can kinda understand because a lot of times, one guy pokes the ball loose, the offensive player tips it, another defender grabs it. But blocks are easy -- if the defender deflects the shot and it doesn't go in, it's a block. |
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gabefarkas
Joined: 31 Dec 2004
Posts: 1313
Location: Durham, NC
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| Posted: Tue Jan 18, 2005 10:28 pm Post subject: |
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| Dan Rosenbaum wrote: |
I am not so worried about the errors in these assist stats. It is a problem, but given the whole host of problems we have to deal with, it seems like a small one to me.
I have tried doing a little of this charting myself recently, and I noticed a few problems.
1. What did you do on breakaway fastbreaks, where there is no possibility of an assist (except in those cases where an assist faciliatates a more exciting dunk)? If we are trying to measure the benefit of an assist, it seems that these attempts should be ignored.
2. What did you do with offensive rebounds that almost immediately led to shots? There usually is no chance of assist on these field goal attempts, so it seems that they also should not be counted.
3. What did you do with foul shots that did not come as the result of a shot attempt? In this case it is unclear whether to count these attempts as assisted or unassisted, since there is no field goal attempt.
Trying to count these cases, I suspect, would result in unassisted field goals generating more points. So if this is what you did, I suspect that the "assist" benefit that you measured might be understated. |
I have to be honest and say that I'm not sure why we would need to parse these out. A basket is a basket; some are assisted and some aren't. Either way, fastbreaks, offensive rebound put-backs and non-shooting foul shots are all a part of the game. If we try to tease these things out of the data, or massage it as such, I feel like it's not really going to be as reflective of what's actually going on.
Does that make sense? |
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Dan Rosenbaum
Joined: 03 Jan 2005
Posts: 541
Location: Greensboro, North Carolina
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| Posted: Tue Jan 18, 2005 11:21 pm Post subject: |
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| gabefarkas wrote: | | Dan Rosenbaum wrote: |
I am not so worried about the errors in these assist stats. It is a problem, but given the whole host of problems we have to deal with, it seems like a small one to me.
I have tried doing a little of this charting myself recently, and I noticed a few problems.
1. What did you do on breakaway fastbreaks, where there is no possibility of an assist (except in those cases where an assist faciliatates a more exciting dunk)? If we are trying to measure the benefit of an assist, it seems that these attempts should be ignored.
2. What did you do with offensive rebounds that almost immediately led to shots? There usually is no chance of assist on these field goal attempts, so it seems that they also should not be counted.
3. What did you do with foul shots that did not come as the result of a shot attempt? In this case it is unclear whether to count these attempts as assisted or unassisted, since there is no field goal attempt.
Trying to count these cases, I suspect, would result in unassisted field goals generating more points. So if this is what you did, I suspect that the "assist" benefit that you measured might be understated. |
I have to be honest and say that I'm not sure why we would need to parse these out. A basket is a basket; some are assisted and some aren't. Either way, fastbreaks, offensive rebound put-backs and non-shooting foul shots are all a part of the game. If we try to tease these things out of the data, or massage it as such, I feel like it's not really going to be as reflective of what's actually going on.
Does that make sense? |
I am very interested in determining the value of an assist. In my plus/minus work, it appears that the value of an assist is fairly high but differs a lot across types of players. I would like to learn more about this, because I think it holds a key towards understanding how to make an offense more efficient (or less efficient if we think of this from the defensive viewpoint).
If the true field goal percentage for assisted true field goal attempts is no different than that for unassisted true field goal attempts, then it can be argued that there is no value to an assist. It would appear that assists do not increase offensive efficiency.
But to make this comparison I need to compare apples and apples and not apples and oranges. Thus, I really only want to count true field goal attempts where there it is realistic for it to be assisted or unassisted. Thus, breakaway layups and putbacks are not really relevant for this analysis. Nor are free throw attempts not stemming from a true field goal attempt. |
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