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Nicolas Batum is generally known as a good defender. Often a very good defender. Occasionally even called an elite defender. He certainly fits the profile, physically. But there's one problem, and it's been bugging me: the +/- numbers disagree. Now normally this doesn't bother me. It's not odd for impact stats to be imprecise. There may be noise effecting things, or the perception of a player may be off. But what confuses me is how strongly the +/- numbers disagree, and how consistently they do so, and how I've completely escaped any explanation to account for this.

Now, pretty much every +/- statistic doesn't merely think Batum is an overrated defender, or an average defender. They think he's a HORRIBLE defender. 4 year RAPM has him at -2.1 xRAPM has him a bit better at -1.7. He's been putrid in raw on/off for a few years now. Now even over a few years, a player can have bad raw on/off numbers if he's consistently playing with another player who is bringing him down. But Batum isn't. The rest of the Blazers' starting lineup, for example, posts much stronger +/- numbers on defense than he does. Last year, for example, he played 2/3 of his minutes with Lamarcus Aldridge. Batum was +4.8 on defense (where positive is bad), while Aldridge was -7.8!

In fact, the only other Blazer who played significant minutes and rivaled Batum's on/off numbers on defense was Jamal Crawford. But he only played about half of his minutes with Crawford, and this year Crawford is gone and Batum's +/- numbers remain the same. I've failed to come up with any explanation for this. I'd assumed it was just noise for a while, but after 3 or 4 years of this being the case, I've failed to come up with any adequate explanation. The only reasonable explanation would be that the Blazers play small ball with Batum in a disproportionate amount of time, but they don't. All the evidence seems to point to one thing: that Batum is just a bad defender.

But this is extremely counter-intuitive. No human seems to think Batum is a bad defender. Even with someone like Serge Ibaka that +/- thinks is generally overrated, there's a not insignificant group of people that see his weaknesses defensively. But in this case, the human element almost unanimously agrees that Batum is a good to very good defender, and the impact statistics basically definitively think he's a bad one. I'm completely unable to make heads or tails of this. Pretty much every other major counter-intuitive +/- phenomena I've been able to explain in one way or another (schematic reasons, noise, whatever). But I have no explanation for this. Is Batum just really overrated defensively for some reason most people simply don't see? The large sample size of data saying that the team is better defensively with him on the bench despite it being better defensively with everyone he plays with on the floor is just hard to ignore.

Sorry for the walls of text.

I haven't seen any Portland games but I'm assuming the reason why most people call him a good defender is because "defensive highlights" like chase-down-blocks, which probably don't give possession to his own team (not saying those are bad, but probably overrated). Chances are high that he also gambles on steals.
There's probably a Kobe-effect here, being that he can defend well when he wants to, but loses track of his man sometimes, doesn't run back on defense as fast as he could etc.
Since he's skinny there's also a good chance he's not good at boxing out

id like someone to explain xRAPM becuase according to that Batum is -1.68 this year, and not just in this instance, but in general i dont get how it differs from traditional RAPM. also doesnt help that his on/off drtg is +7.6 (from 108.5 with him on the court to 100.9 with him off). i do think batum is a good defensive player but both his on/off and +/- numbers seem to be poor, and its not like portland has much of a bench to speak of.

so yeah i realize my post is the tl;dr version of OP's post so im with him on this.

I'm afraid I don't have a real answer to the question in general, but as far as xRAPM vs RAPM:

The "R" in RAPM stands for Regularized, which is another way of saying "we don't trust the data that's too much of an outlier here particularly when sample size is small, so we're going to bias the data back toward the norm we'd expect to happen".

For the most vanilla RAPM that's essentially regression to the NBA's mean, but there's no reason it has to be that way. With his xRAPM stuff, the most eyebrowing-raising thing that he (Engelmann) is doing is using an expectation for a player that factors in how impressive his box score stats are. This makes some sense given that we do have expectations that star players have more pull on the scoreboard and thus their +/- data is going to be less prone to randomness than other players, and Engelmann's main claim that I can agree with is that if you were only going to use 1 stat ever it would be better to use xRAPM than RAPM.

Of course it's stupid to only use 1 stat and the new additions compromises the orthogonal relationship between +/- and box score data so I'm not a big fan.

Well, if we look at Synergy numbers we see him being rather bad in terms of defending spot-up shots. Those shots are highly efficient and Batum is neither forcing turnovers a lot in those situation nor is he able to make the shooter miss many of those shots. That is a scheme seen throughout his career. The opponents are shooting better, especially from longrange, when Batum is on the court. Also, the defensive rebounding gets worse. It might be possible that Batum's concentration on 1on1 situations even off-ball is distracting him from getting into the right spots in terms of rebounding.
Another point would be transition opportunities due to turnovers. Batum's turnover rate is really high, and given his position on the perimeter such turnovers are leading to more opportunities for the opponents in transition. He may add a couple of chase-down blocks here and there, but overall it helps the opponents to create easier opportunities.

His individual 1on1 defense is excellent, but a player is not just affecting the team's own defense with that.

I like mystics turnover theory. When Monta Ellis was with the warriors he was a turnover machine, and despite the fact that no one has ever thought him a strong defender, I think it was really the turnovers that pushed his defensive rapm though the floor.

But Batum's turnover proneness only started this year, while his bad on/off numbers go back at least two years before this year.

Poor defending of spot ups and poor boxing out might explain it though.