MAVS 108, PELS 92

[mavsluvr]

It tells you how the TEAM (which includes that player) played/performed when that player was on the court.

No, it doesn't. It tells you what the score differential was during any particular time period. It tells you absolutely nothing about how the team performed during those periods. Doesn't tell you whether they shot well or poorly; whether they were active on the glass or not; not whether they were encumbered by injuries; not whether the officiating was of good or bad quality; not the quality of the opponents on the floor during any time segment. If the team as a whole has a +/- of -5 (meaning they lost the game by five points), it tells you nothing about whether they played well or poorly. If they have a +5, it doesn't tell you anything about whether they played well or poorly. It simply gives you a point differential. 

That is the fact it presents.

The fact it presents is the point differential -- NOT how any particular lineup played, or how well they played. 

For instance, The Mavs have been severely outscored this year when Luka has played. They have on the other hand the Mavs have handily outscored their opponents when Luka hasn't played. That is the fact which +/- is telling us and it won't and cannot "lie."

It actually doesn't tell us that. There have been times when Luka was on the floor when the point differential was positive. There have been other times when it was negative. In the cumulative total over ten games, it is apparently negative. That doesn't tell you that Luka has played worse than the guys who played without him. It doesn't tell you anything about how he played, or how any of the other nine players who were on the court when he was played. The performance of  the various groups of ten on the court when he wasn't there tells us nothing about whether Luka played the right way, the wrong way, was showing progress, was a total dud, or anything of the sort. It certainly doesn't tell us that the players with better +/- numbers played better than he did. 

I think that is where we may be attempting to make a leap that is not logically valid. That stat tells us a factual piece of information -- what the point differential was. We cannot logically extrapolate anything at all about the players' performance from that number. 

It is what it is. Our conclusions beyond that are 100% debatable, but with larger sample sizes our confidence in reaching more conclusions will get higher. 

But we also have lots of other numbers and observations to add to what +/- is telling us. For instance we see Luka shooting 29.5% from three. We see the team as a whole shooting 26.7% on OPEN threes. So for instance we can safely surmise that Luka's -2.5 offensive on/off is partly (mostly?) from Luka shooting poorly himself AND from his teammates missing so many of the open looks he has gotten them.

We can tell what percentage of shots Luka is making from watching him and from looking at the field goal percentage stats. We cannot tell anything about how well he or his teammates are shooting or from looking at his +/- stat. It is possible to shoot very well, score a lot of points, and still have a negative +/-, and vice versa. 

The stat tells us the truth about point differentials, as you point out. It tells us nothing else. Whatever surmises we make based on it are matters of interpreting that data. We may be able to use it as one of a number of tools in our evaluation box. But it's not some Delphi-like oracular stat that constitutes some kind of infallible information about how a person, or a lineup, or a team, plays.