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APBRmetrics
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bchaikin
Joined: 27 Jan 2005
Posts: 681
Location: cleveland, ohio
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 Posted: Sun Feb 15, 2009 2:05 pm Post subject: |
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"...I don't understand why you're having so much trouble understanding the concept behind on/off +/- metrics... It's really not a difficult concept to grasp..."
the concept of on/off +/- is quite simple, it's some of the results of the adjusted +/- that i question...
if a rating of a metric for just a few players out of the enitre league is off because of some quirk of the metric (any metric, not just adjusted +/-), that's understandable. there are going to be outliers in any system. but in this case the authors of adjusted +/- are vehement in their characterization of chris paul being a liability on defense in 07-08, despite considerable evidence to the contrary...
are you also of the opinion that chris paul was a liability on defense in 07-08, despite what actual evidence (steals) and inferred evidence (hornets opposing PG eFG% when paul did/did not play) shows and what the coaches and media claim?... |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Sun Feb 15, 2009 2:24 pm Post subject: |
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| bchaikin wrote: | ...this piece of information strongly suggests that Chris Paul is playing far better defensively.
to whom? to you? the hornets team defense gets a bit better (not insignificant, but not significantly better) when paul was on the floor from one season to the next (107.6 to 106.1 pts/poss allowed), but because of something that has absolutely nothing to do with chris paul himself, and based on a sample size of data that is just 1/3 the size of the data that when paul played and is thus much more apt to be skewed, adjusted +/- comes to the conclusion that paul "...was actually a liability on defense..." one season (07-08 ) but is "...far better defensively..." the next (08-09)?...
this despite the fact that the defensive pts/poss allowed when he actually did play changed little? and despite the fact that the 07-08 season that adjusted +/- finds paul to be a liability on defense there are many others who have found his defense to be quite good? after all he received not only the most votes among all PGs for the all-D team, but also the 7th most votes among all players in the entire league for DPOY? and this from the people that have actually seen him play alot - how much did you actually see chris paul play last year?...
tell us then - knowing that the number of steals paul got both seasons is about the same (and each is a defensive stop), and the data for eFG% allowed seems to show at most 1 additional defensive stop through missed FGAs forced every 2 games, just what exactly has chris paul - himself - done this year that he did not do last year such that he is "...playing far better defensively..."?...
do you have any evidence whatsoever that paul has forced significantly more non-steal turnovers, or that he was personally responsible for forcing more missed FGAs by the players guarded by his teammates (that might not be reflected in data such as that found on 82games.com) this year than last year?...
"...a substantial drop in the effectiveness of Paul's opposing PGs in 08-09. Bchaikin attempts to dismiss this latter point, waving away, for example, the impact of the drop in eFG% - down to 46.7% from last year's 51.8% - by calculating a net of roughly 36 more defensive stops per season, but he ignores the fact that the bulk of a player's defensive efficiency lies in his contribution to team defense - rotations, double-teams, transition coverage, traps, etc.; if a player's on-ball defense is improved, presumably it's not a stretch to infer that his off-ball defense has experienced commensurate improvements, as well;..."
oh, i get it. let's just completely ignore the actual data that does in fact exist, and then infer what we will to validate our conclusions. i'll ask again, how much have you actually seen chris pual play? because in the opinions of those who have seen him play the most (the coaches and media) he was a very good defender in 07-08...
"...bchaikin might also ponder what Paul's opposing PGs would have yielded in collective FG% this season had the Hornets benefited from the 07-08 version of Chandler...."
oh, so now not only something that happened when chris paul was not on the floor, but something that has not happened period, is futher validation of adjusted +/- conclusions?... |
Bob,
I've already explained, at some length, several converging lines of evidence that support the validity of Paul's defensive APM improvement from last season to this. Given the increasingly sarcastic, snarky tone of your posts, I hope you'll understand if I fail to repeat the exercise or to respond further. Time is precious. |
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Neil Paine
Joined: 13 Oct 2005
Posts: 774
Location: Atlanta, GA
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| Posted: Sun Feb 15, 2009 2:25 pm Post subject: |
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| bchaikin wrote: | | are you also of the opinion that chris paul was a liability on defense in 07-08, despite what actual evidence (steals) and inferred evidence (hornets opposing PG eFG% when paul did/did not play) shows and what the coaches and media claim?... |
I wouldn't have used the term "liability", no... That implies that somehow his defense is bad enough to offset his offense, which would basically be impossible given his individual box score stats, his high % of team minutes and NOH's great point differential. Was he overrated because of the steals, though? Probably. I distrust steals as a primary defensive metric on principle -- you can pad them by gambling at the team's expense. CP3 is far from the worst offender at this, but it's always a concern. Subjectively, I'd say he was somewhere above average for PGs, maybe not 2nd-team All-Defense caliber, but he's got good strength, quickness, a high basketball I.Q., though his size is a liability. I have no numbers to back that up, just what I've seen of him play in college and the pros. |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Sun Feb 15, 2009 2:34 pm Post subject: |
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| davis21wylie2121 wrote: | | Ilardi wrote: | | My primary goal in APM analyses is to provide a metric of a player's actual contribution to the team's bottom line in a given season, not to provide an indicator of a player's potential contribution (e.g., if he were to start playing harder, adopt a different role in a different system, etc.) ... as Bastillon implies, it would be a mistake to look at a player's defensive APM number from a given season and conclude: this is a valid rating of Player X's defensive ability. Rather, it's merely an indicator of his defensive contribution over the period in question. |
Personally, I like APM, and I believe there's a lot of potential there as APBRmetrics moves into the future. But I guess the question a lot of people would have, given what I quoted, is "what's the point of APM?" I mean, if I'm a GM, I'm probably not asking how good a player was in the past, I'm asking how good he is now -- and more importantly, what can I expect out of him in the future?
This is one of those "value vs. ability" questions we always run into, and I've found that while "ability" is the real practical concern for front offices, "value" is one of those almost purely academic exercises regarding past players that's fun for discussion, but of little use to decision-makers in the here and now (besides, in APM's case, it can't even describe the past value of players before PBP was widely available).
So I suppose the question is, if APM doesn't really try to predict future performance, and it can't tell us about past performance except over the past handful of seasons, doesn't that kind of limit its utility? (Just playing devil's advocate here, btw) |
Davis,
Thanks for the interesting questions.
Of course, I think you could ask them of any metric: they all describe what has happened, not what will happen. That being said, despite the moderate degree of year-to-year fluctuation in defensive APM, there's still a great deal of stability on balance (e.g., KG is still a highly-rated player this year on defensive APM, as are Camby, Chuck Hayes, Yao, Kirilenko, Aldridge, etc.).
In a nutshell (as we psychologists like to say): the best predictor of future behavior is past behavior. A corollary: the best predictor of future offensive/defensive APM is past APM. Thus, knowledge of a player's recent offensive and defensive APM ratings should be highly useful to front-office decision-makers. But they'll want to keep in mind that there's probably lots of defensive upside for poorly rated defensive APM players if they happen to be highly athletic and possessed of high basketball iq's (and are lucky enough to play for a coaching staff that can motivate them).
Last edited by Ilardi on Sun Feb 15, 2009 2:46 pm; edited 1 time in total |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Sun Feb 15, 2009 2:42 pm Post subject: |
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| Kevin Pelton wrote: | | Ilardi wrote: | | What I'm discovering is that offensive APM (which correlates quite heavily with traditional boxscore stats, and metrics like PER that are derived from them) is more stable from year to year than is defensive APM. This isn't terribly surprising, but I think it's an important discovery with myriad implications. It suggests, for example, that many players have the potential to greatly increase their on-court impact by giving greater effort (and playing smarter) on the defensive side of the ball. |
Wouldn't this also be consistent with the conventional-wisdom thinking that coaches have a greater impact on the defensive end than on offense and that APM is crediting players for the work of their coaching staff?
| Quote: | | In other words, using multiple seasons' worth of data, we can still get low-noise estimates of each player's performance in any given season, despite the fact that some players will vary quite a bit on the defensive end from year to year. |
I think this response tends to understate potential aging effects in the data. For someone like, say, Michael Finley, including numbers from several seasons ago when he was still relatively in his prime doesn't seem to help us understand his ability at this point in his career. |
Kevin,
Yes, I agree that the best coaches/staffs are the ones able to motivate their players to greater effort on the defensive end, and thus - as you suggest - player defensive APM ratings likely reflect (to an unknown extent, but one that's non-trivial) the impact of coaching. It's ultimately an empirical question, and one I hope to address in future models that include variables for the effect of coaching staffs (above and beyond player effects).
I also agree with your basic point about "maturation effects" (declining performance in later years, improving performance in early years), over time in multi-year datasets. However, given that prior seasons are weighted much less heavily than the target (i.e., current) season in any multi-year APM model, the net biasing effect of such maturation processes should be modest.
Nevertheless, I've toyed with the idea of using time-trend analyses (i.e., latent growth curve models) to model each player's APM trajectory over time, which should help smooth out some of the year-to-year noise and (hopefully) eliminate some of the bias that you've identified. |
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Neil Paine
Joined: 13 Oct 2005
Posts: 774
Location: Atlanta, GA
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| Posted: Sun Feb 15, 2009 2:45 pm Post subject: |
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Thanks. I suppose my main concern was along the lines of Kevin's earlier post about aging -- maybe you reduce the standard error by including as many past seasons as possible, but how much does the inclusion of somebody's 2003 season help us understand how good he is now? I can see a lot of instances where it might actually make the estimate of his "true ability" worse, especially for older players.
On a related note, are the weights of past seasons in the multiyear APM geared to produce the lowest standard errors, or to best predict next year's APM? Or is there a difference? |
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deepak
Joined: 26 Apr 2006
Posts: 664
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| Posted: Sun Feb 15, 2009 3:17 pm Post subject: |
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My intuition tells me that the truth about a player, or team, usually lies somewhere between the average and what the past numbers tell us. So, if a team wins 60% of its games half-way through the year, they'll probably win between 50% and 60% the rest of the way.
Let's suppose that idea applies here (it may not, not sure), and we have a player who rates very strongly defensively with, say, a +8 per 100 possessions. Then wouldn't it be more likely his actual defensive contribution is less than +8 instead of greater than +8?
If that thinking is correct, than to really get at the overall contributions of a player based on APM, we should probably weight his offense more than his defense because offense is less volatile (and, so, less likely to tend to the middle). So, if a player is a -8 on offense and a +8 on defense, he's more likely to be a net negative than a net positive.
Does that make sense? |
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bchaikin
Joined: 27 Jan 2005
Posts: 681
Location: cleveland, ohio
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| Posted: Sun Feb 15, 2009 3:51 pm Post subject: |
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I wouldn't have used the term "liability", no... That implies that somehow his defense is bad enough to offset his offense, which would basically be impossible given his individual box score stats, his high % of team minutes and NOH's great point differential.
exactly, and that is what a few others are failing to see...
let's take a look at the on/off +/- for chauncey billups. the nuggets on/off pts/poss allowed on defense with/without billups this year is 108.0/103.4, and billups has played 2/3 of the team's minutes at PG and leads the team in minutes played. are we to come to the conclusion that billups is not a good defender based on this? despite the fact that denver as a team has the league's 6th best/lowest defensive pts/poss allowed? i sure wouldn't...
also take a look at detroit's on/off pts/poss allowed on defense with billups for 06-07 and 07-08 when the pistons were 7th and 4th best in the league in defense. its 105.9/103.3 for 06-07 and 104.3/103.2 for 07-08, with billups again playing close to 2/3 of the team's minutes at PG. so for not 1 or 2 but for 3 straight seasons the teams that chauncey billups has played for have been better on defense without him than with him. are we to come to the conclusion that he has been a poor defender based on this data? i sure wouldn't...
if next season the nuggets are much worse on defense for the 1/3 to 1/4 of the time billups does not play, but similar or slightly better with billups, yet the team defense overall is about the same as this year (like chris paul and the hornets in 07-08 and 08-09), should we then come to the conclusion that billups was a much worse defender in 08-09 than in 09-10? again i sure wouldn't...
I've already explained, at some length, several converging lines of evidence that support the validity of Paul's defensive APM improvement from last season to this.
and conveniently ignored evidence that is contrary to your conclusion...
for example paul's high steals. last year he got 217 and this season he is on pace for a similar number. a steal is a defensive stop, a blocked shot is not. a blocked shot has to be rebounded for it to be a stop. let's say for example that about 60% of all blocked shots are rebounded by the defense. so 217/.60 is about 360 blocked shots that would be needed to get as many defensive stops as 217 steals. yet you are calling paul a liability on defense one season and a heckuva defender the next, with the same amount of steals. are there any shot blockers who have gotten as many as 350 blocked shots two seasons in a row (heck even just 300) that you that would consider were a liability on defense one season but a "...far better..." defender the next?...
the fact is that statistically speaking you are grossly undervalueing the impact of paul's high steals...
Given the increasingly sarcastic, snarky tone of your posts...
sarcastic? because you refuse to debate the evidence contrary to your findings? snarky? because i ask you if you have actually seen chris paul play, when those who actually saw him play on a consistent basis in 07-08 (coaches/media) voted that he was a top defender when your methodology presents results at the opposite end of the spectrum?...
did you or did you not open this thread with this comment:
Chris Paul's defensive rating is dramatically higher than it was last year (improving from -4.54 to +6.78 this season)... I'd be curious to hear your thoughts on what (if anything) might account for this marked improvement, especially from those of you who have watched the Hornets extensively for the past couple seasons...
if you are looking for open dialogue as to the perceived pros and cons of your adjusted +/- methodology based on its findings that you have presented, then this is the place. if however you are only after praise and accolades, then...
I hope you'll understand if I fail to repeat the exercise or to respond further.
i understand... |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Sun Feb 15, 2009 8:20 pm Post subject: |
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| davis21wylie2121 wrote: | Thanks. I suppose my main concern was along the lines of Kevin's earlier post about aging -- maybe you reduce the standard error by including as many past seasons as possible, but how much does the inclusion of somebody's 2003 season help us understand how good he is now? I can see a lot of instances where it might actually make the estimate of his "true ability" worse, especially for older players.
On a related note, are the weights of past seasons in the multiyear APM geared to produce the lowest standard errors, or to best predict next year's APM? Or is there a difference? |
Great questions. Whenever we derive APM estimates, there's going to be an interesting trade-off: on the one hand, we always want estimation errors to be as low as possible, while on the other hand we typically want to know how each player is performing right now. If the only issue were minimizing errors, we would include as many seasons as possible, and give them all equal weighting (differential weighting across seasons actually adds non-trivial noise). But if we care about how each player is doing this season (or next), we clearly don't want to give equal weighting to past seasons. And yet if we drop all past seasons entirely from the model, error terms get so large that the resulting estimates are not terribly informative.
So, I've experimented with many different approaches, but all involve including prior seasons for noise reduction, but giving them such low weightings (compared with the present season) that they don't introduce enormous bias into the estimates for players that were much better or much worse a few years ago.
Still, I think the next step will involve generating a multi-year APM profile for each player - separate offense/defense APM estimates for each season over a 6+ year span - and then projecting each player's ensuing "growth curve" forward (adjusting for age, position, experience, etc.) in order to more accurately project the following season's performance. I guess that'll be the next article you see from me on 82games . . .
Last edited by Ilardi on Mon Feb 16, 2009 12:04 am; edited 1 time in total |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Sun Feb 15, 2009 10:26 pm Post subject: |
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| deepak_e wrote: | My intuition tells me that the truth about a player, or team, usually lies somewhere between the average and what the past numbers tell us. So, if a team wins 60% of its games half-way through the year, they'll probably win between 50% and 60% the rest of the way.
Let's suppose that idea applies here (it may not, not sure), and we have a player who rates very strongly defensively with, say, a +8 per 100 possessions. Then wouldn't it be more likely his actual defensive contribution is less than +8 instead of greater than +8?
If that thinking is correct, than to really get at the overall contributions of a player based on APM, we should probably weight his offense more than his defense because offense is less volatile (and, so, less likely to tend to the middle). So, if a player is a -8 on offense and a +8 on defense, he's more likely to be a net negative than a net positive.
Does that make sense? |
You're referring to an effect often referred to among statisticians as "regression to the mean" - i.e., the fact that outlier (high/low) measurements typically move closer to the mean when re-assessed.
Ultimately, the degree to which such regression-to-mean effects occur on offensive vs. defensive APM estimates is an interesting empirical question (one readily capable of being addressed), but I suspect you're probably on to something with the suggestion that the regression will be greater for outlier defensive vs. offensive APM values. |
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cherokee_ACB
Joined: 22 Mar 2006
Posts: 157
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| Posted: Mon Feb 16, 2009 10:01 am Post subject: |
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| Ilardi wrote: | | If the only issue were minimizing errors, we would include as many seasons as possible, and give them all equal weighting (differential weighting across seasons actually adds non-trivial noise). |
The noise in adjusted ratings have, in my view, two components: an error per-position and team, and an individual noise component. Including multiple seasons mainly reduces the first one, but has little impact on the individual component.
Case in point: Paul and Pargo. Let's compare their single season (1S) adjusted ratings with their ratings derived from multiple seasons (MS)
| Code: |
Paul Pargo Diff
Off 1S 10.8 0.3 +10.5
Off MS 9.2 -1.1 +10.3
Def 1S -10.4 -4.3 -6.1
Def MS -4.5 0.5 -5.0
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When Steve and Aaron added several seasons, it moved the ratings of both players up & down, but the difference remained stable. The single season correlation is not much sure about the defensive strength of New Orleans PGs, but it can tell pretty well how much better Pargo was.
What can we do about this? For once, we can try to separate the two noise components, by deriving the individual noise contribution using probability theory. Or we can tell the correlation to attempt to find the difference between starters and bench players of a team, instead of the rating for each player. We'd compute the coefficients for Paul and for (Paul-Pargo), which should give a good estimate (i.e. with a low error) of the adjusted difference between both players. |
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Ilardi
Joined: 15 May 2008
Posts: 262
Location: Lawrence, KS
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| Posted: Mon Feb 16, 2009 12:48 pm Post subject: |
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| cherokee_ACB wrote: | | Ilardi wrote: | | If the only issue were minimizing errors, we would include as many seasons as possible, and give them all equal weighting (differential weighting across seasons actually adds non-trivial noise). |
The noise in adjusted ratings have, in my view, two components: an error per-position and team, and an individual noise component. Including multiple seasons mainly reduces the first one, but has little impact on the individual component.
Case in point: Paul and Pargo. Let's compare their single season (1S) adjusted ratings with their ratings derived from multiple seasons (MS)
| Code: |
Paul Pargo Diff
Off 1S 10.8 0.3 +10.5
Off MS 9.2 -1.1 +10.3
Def 1S -10.4 -4.3 -6.1
Def MS -4.5 0.5 -5.0
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When Steve and Aaron added several seasons, it moved the ratings of both players up & down, but the difference remained stable. The single season correlation is not much sure about the defensive strength of New Orleans PGs, but it can tell pretty well how much better Pargo was.
What can we do about this? For once, we can try to separate the two noise components, by deriving the individual noise contribution using probability theory. Or we can tell the correlation to attempt to find the difference between starters and bench players of a team, instead of the rating for each player. We'd compute the coefficients for Paul and for (Paul-Pargo), which should give a good estimate (i.e. with a low error) of the adjusted difference between both players. |
Thanks, Cherokee - that's an intriguing thought. I think I'd be inclined to try teasing apart these different sources of noise in an HLM (hierarchical linear model), using team/position as superordinate grouping variables and player as lower-level variables. The problem is that such models can get really messy and unwieldy, and they don't always reduce noise the way one would hope. Maybe you've got some thoughts on a better way of going about it (Bayesian estimation, etc.)?
By the way, I'm not convinced that the Paul/Pargo example you provided is completely representative of the way APM always treats starter-backup pairs. Just eyeballing other pg pairs last year like Rondo/House, Bibby/Law, and Billups/Stuckey . . . I didn't see quite the same consistency in (starter - backup) across single-year and multi-year estimates. If you (or someone else on APBR) has time to look at this systematically, I'd be highly curious to know if it's actually a stable phenomenon. |
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cherokee_ACB
Joined: 22 Mar 2006
Posts: 157
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| Posted: Mon Feb 16, 2009 2:20 pm Post subject: |
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| Ilardi wrote: |
By the way, I'm not convinced that the Paul/Pargo example you provided is completely representative of the way APM always treats starter-backup pairs. Just eyeballing other pg pairs last year like Rondo/House, Bibby/Law, and Billups/Stuckey . . . I didn't see quite the same consistency in (starter - backup) across single-year and multi-year estimates. |
Of course, I choose a convenient example, but there are others. For Rondo/House and Billups/Stuckey playoff data did have an impact which Eli's numbers didn't catch (this can be seen in bv.com ratings pre and post playoffs); for Bibby/Law I don't have a quick explanation. Anyway, in all cases the individual ratings tend to move more than the rating difference.
Regarding hierarchical models, I'm not familiar with them. Bayesian estimation could be useful to deal with reference players, or as a way to mix statistical and adjusted +/-, but I haven't given many thoughts to it. |
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bastillon
Joined: 04 Nov 2008
Posts: 55
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| Posted: Mon Feb 16, 2009 3:33 pm Post subject: |
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bchaikin,
1. Mentioning players/coaches opinions without their arguements makes no sense. It's not important what they think, but why. They didn't give any reason. For example Kobe Bryant is allegedly, according to coaches/players nominations/awards the all-NBA caliber defender through the whole last decade. Even though some of his seasons(especially after Shaq has left him) he didn't really bother by defending opposing players or giving any effort whatsoever so he could save his energy for the Lakers' poor-at-the-time offense. Some awards are given for the hype or names. We have to be careful judging by that. League tend to overrate players for some reason - maybe it's the numbers, maybe names, maybe conditions, anyway it's a fact and there are many examples you certainly know. We can just look at the Hoopshype's salaries and see how many overrated players there have been in the NBA. So my point is that it doesn't matter who says, but what says.
2. Players recording high number of steals are often overrated. That's exactly what happened to Paul last season. There's no way he could be 7th best defender in the league. Not even close.
First off, small guys don't have that kind of influence on team defense that bigs have. Not surprising it's big who are leading in defensive categories and also having much more success getting awards. It happens hardly ever that elite guard could have impact on team defense good big man has. If you have the team with dominant big men inside and poor defenders at 1-3 positions, your team defense could get away with it, because even if your small guys are matador-type players, there's always that 2nd line of defense. Second part is rebounding, you can have Kidds, Rondos all you want, but without rebounding bigs your team is weak on the boards anyway - just look at the Kidd's-era Nets. Given that defending the paint and rebounding are two of the crucial elements of team defense, small guys have much less effect on it(stats also confirm it).
If so, Paul not only must have been one of the elite guard defenders(which he wasn't) he must have been better than most of the good defensive big guys. Would you really make a case that Paul was better than Garnett, Duncan, Camby, Okafor, Howard, Yao, Sheed ? I don't think you would.
Being a realist, Paul wasn't even close to deserve 7th place on DPOTY list and even him being a candidate is pretty ridiculous.
That's about theory...
3. You're making good points about the evidence but you don't take into account stats not showed in the boxscore. You count the steal as a defensive stop as if gambling wasn't causing total defensive breakdowns. We can only suppose how many steals per failed gamblings Paul has had, but it's sure he had some. Only then we can trust your evidence based on steals area.
Blocks maybe don't count as a defensive stops, but they intimidate from attacking the paint. Shot blocker over gambling little guy stealing the ball any day of the week.
Besides, it's not only missed shots and turnover player can cause. Being a defender you care about staying in front of the opponent first, not to draw help from your teammates, not letting the defense collapse.
Then there is pick and roll defense, which is particularly important as for as PGs concerned.
Then there are rotations - how quick you are, how hard you contest.
You simply eliminate many of the important factors to team defense in order to make your points look more legititimate. On purpose or not, it's not fair. |
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gabefarkas
Joined: 31 Dec 2004
Posts: 1312
Location: Durham, NC
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| Posted: Mon Feb 16, 2009 4:23 pm Post subject: |
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| davis21wylie2121 wrote: | | bchaikin wrote: | | the hornets team defense gets a bit better (not insignificant, but not significantly better) when paul was on the floor from one season to the next (107.6 to 106.1 pts/poss allowed), but because of something that has absolutely nothing to do with chris paul himself, and based on a sample size of data that is just 1/3 the size of the data that when paul played and is thus much more apt to be skewed, adjusted +/- comes to the conclusion that paul "...was actually a liability on defense..." one season (07-08 ) but is "...far better defensively..." the next (08-09)?... |
At the same time, I don't understand why you're having so much trouble understanding the concept behind on/off +/- metrics, Bob... When a player is on the court, the team's DRtg is not only a function of his ability, but that of 4 other guys. So how do you isolate his specific contribution? You find instances in which some combination of the other 4 guys are on the court, and the player we're studying isn't. Is the team's defense better? That suggests the other 4 guys were contributing more to the team's DRtg when our guy was on the court (which is essentially what happened in '08: NOH was 6.2 efficiency points worse on D when Paul was in the game vs. when he wasn't). Or is the defense a lot worse? That suggests our guy was actually making a far bigger contribution than the other 4 guys when he was on the floor (like Paul in '09: NOH is 5.7 DRtg points better when he's on vs. off). It's really not a difficult concept to grasp on paper -- the hard part is getting around the fact that players often play with generally the same group of players, because it's tough to isolate one specific player vs. the other 4 guys on the court with him when he's always playing with the same 4 guys, and they're always sitting when he's sitting. |
Part of the problem lies in the fact that it's not really a true plus/minus statistic like raw +/- is. Raw +/- measures the difference on both ends of the court. Once you start separating it out into "offensive" and "defensive" components, it ceases to be an interpretable +/- and instead it becomes just a "plus" (Offensive) and a "minus" (Defensive), separate from each other. |
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