SideshowBob
Rookie
- Joined
- Sep 15, 2015
- Messages
- 33
- Reaction score
- 88
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Actually his plus/minus is not so extreme. The reason it looks so high is that Blatt is playing Lebron with other starters, specifically Kevin Love, and putting out horrible bench lineups when Lebron is off court.
Yes but this is inherent, this is always generally the case, with almost every star player. And the fact that there are horrible bench lineups playing out there with no Lebron/Love is exactly the point.
Real adjusted plus-minus, which adjusts for the other players on the court with Lebron, doesn't look so extreme or really even all that great for him this year. Lebron's RAPM is 6.43 this year, 4th in the league. Last year it was 8.78 (2nd in the league) and the year before with Miami it was 9.08 (first in the league). So with proper adjustment Lebron's plus-minus is actually kind of low for him.
RPM is not the same as RAPM. Here is the explanation I posted on RealGM:
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[#1]APM is Simple OLS, exactly as Spaceman described it. Set up every 5on5 matchup, set equal to the scoring margin and solve for each player across the league (I've run it for a few years, its like ~65000 lines of 5on5 matchups). The resulting coefficients (on each player) are the APM values. This needs a very large sample size to say anything of considerable meaning; a single-year APM has large error terms on each coefficient, multi-year (usually 2-year) studies are preferred.
RAPM is essentially the same thing (OLS) with one exception. It introduces what we'll call a "reference matrix", basically each player is given a baseline value, towards which their coefficient will be pulled. I believe this tries to reduce the multicollinearity problem.
In [#2A]vanilla/basic RAPM, every value in the reference matrix is set to 0. The greater the amount of games played, the less weight that reference of 0 has. It is almost the same as APM [#1], but the regression towards 0 in theory reduces the error within a single-season set. It's still fairly volatile, but it's better than APM [#1] is within a single year. There is also [#2B]multi-year RAPM, which just uses a larger number of seasons, with most weight given to the current year and less and less weight given to previous years, reference matrix of 0s.
[#3]Prior-informed RAPM is essentially the best (ITO out-of-sample prediction) version of this family without introducing the box-score. It's built the same way as RAPM, but the reference matrix uses RAPM values from the previous year, instead of all players being set at 0. Again, as the sample size of the season grows, the reference value holds less and less weight. Obviously this only works once we have multiple years of data, in the 1st year, there is no prior. In the 2nd, we have a prior but it is vanilla RAPM [#2A], but by the 3rd year we can use PI RAPM of the previous year to inform the current year.
[#4A]RPM is RAPM, but the reference matrix is made up of SPM values (SPM is again, regression of box-score metrics on a multi-year non-box-score model such as RAPM). There is also [#4B]multi-year RPM, which is the same as multi-year RAPM, except it presumably uses a reference matrix of multi-year SPM values.
There is also [#5]prior informed RPM. Again, same idea as PI RAPM [#3], single-year, reference matrix of prior-year's RPM values.
ESPN, is unfortunately poor at labeling and differentiating between what they put up on their site. Luckily for us, JE, one of the co-creators, often posts on APBRmetrics and has offered clarification.
ESPN's site has RPM for three seasons, 2014, 2015, 2016.
2014 is multi-year RPM [#4B]
2015 is single-year RPM [#4A]
2016 is single-year RPM [#4A]
It's silly, but they've given us 2 different models for 3 years and labelled them all the same.
Multi-year RPM [#4B] for 2015 was provided in an article last year before the playoffs. Other than that, everything they've posted is single-year. I asked Engelmann for multi-year RPM for this year on twitter, and he said that it was in the works. Where it will be posted I can't say, but I'm sure he'll clarify at some point.
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The reason Lebron's RPM [#4A] (which is what's currently +6.43 on ESPN) is low right now is that the SPM prior from the reference matrix is holding most of the weight, which, for the sake of our conversation, is the equivalent of PER/BPM that we see on BBR. Essentially, his low box-score line is holding his RPM [#4A] rating down. As the sample size of 2015 games increases, his strong +/- performance will drive that up, as it will gain weight relative to his box-score line (which should also improve as his shooting normalizes).
ALSO, since I made that post, JE has updated with multi-year RAPM [#2B]. No prizes for guessing who's leading there. To reiterate, this set is data from the last few years, with most weight placed on the current season, and then consecutively less weight on each prior season (probably back to 2013 or 2014), with a reference matrix filled with values of 0 for each player.
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