Henry at TrueHoop had a post today on the Celtics and offensive rebounding that I think is interesting for a variety of reasons. One interesting point is that the Celtics were the worst offensive rebounding team in history; any time a recent team is the best/worst anything in history, it’s kind of interesting. The strategic idea of going for offensive rebounds is also interesting. But I’m not going to chat about that one today. What I thought was most interesting was how Henry jumped from one measure, offensive rebounding, to another, shots per possession, and in the process completely messed up his thinking.
So, like we said, the Celtics were the worst team in history at offensive rebounding, and thus obviously the worst team last year. Also in the John Hollinger article that Henry is working from is the tidbit that the Celtics were last in the league in shots per possession. That also sounds bad. But what Henry notices is that teams 2 and 3 on that list are the Thunder and Heat. They obviously did pretty well overall. So now Henry says that some of the best teams in the league don’t emphasize offensive rebounding, and then he’s off to talking about rebounding and defense and the Blazers. But what was skimmed over is that shots per possession is not offensive rebounding.
A possession can end in one of three ways: a defensive rebound, a turnover, or a made shot. So if a team were to have a low number of shots per possession, it could do it in one of three ways. One would be to turn the ball over a lot. Good teams obviously don’t do this for the most part, although we can turn to bball reference and see that the Thunder actually were bottom in the league at turnover percentage. The Heat and Celtics were also below average, as it turns out. So all three of these teams happened to hurt their shots per possession by simply not getting a shot off more than most other teams. The second way, as Henry has covered, is to not get offensive rebounds. If you get an offensive rebound the possession continues, and so you can get extra shots on the same possession. That same bball reference page tells us that the Thunder were in fact above average and the Heat average at offensive rebound percentage. So they were not hurting their shots per possession because of rebounding. The third way (and perhaps more important?) way to reduce your shots per possession is simply to make your shots. If you make it the first time, you only get one shot per possession. And if you turn to Hoopdata and sort by true shooting percentage, you’ll see that the Thunder and Heat were first and fourth in the league at making their shots last year.
So to sum up: offensive rebounding is only one piece of shots per possession. As it turns out, the Heat and Thunder were perfectly competent at offensive rebounding. Instead they were good at shooting and bad at holding on to the ball, and that is probably what put them at the bottom of the shots per possession list. The Celtics were 8th at shooting, which would also limit their shots, and 6th worst at turnover percentage. Put that all together, and you have the worst team in the league at getting shot opportunities.
Henry’s mistake reminded me of another one in an old article by Neil Paine (here if you have insider). He was talking about how the Celtics were increasingly running their offense through Rondo and how that was a bad idea. He notes that Rondo was third in the league in pure point rating and that his touches per minute were second in the league to Steve Nash. Obviously Rondo had the ball a lot and was a big distributor. But then Paine points out that as Rondo’s usage increases, the Celtics tend to be worse on offense and lose more games. But do you see what happened there? Paine moved from touches and point rating to usage; usage doesn’t include assists. If Rondo had a relatively high usage rate in a game (or season), it means he either had more shots or more turnovers. Turnovers are always bad, and Rondo is a poor shooter so more shots is also bad. Those are not good things for the Celtics.
So was it bad for Rondo to have more usage? Probably. Was it bad for the Celtics to run the offense through Rondo? Probably not. The raw on/off numbers say that Rondo was a huge plus for the Celtics, Rondo’s offensive RAPM was low but positive, and he made the Celtics 7 points better on offense than his main line-up counterpart. It’s tricky to parse these things out and line-up data isn’t perfect, but if we had to guess it seems like Rondo helps the Celtics offense on the whole. But he apparently does it with ball movement, which doesn’t show up in his usage (in fact, it measures his ball stoppage to some degree). By making a point with one set of numbers and then shifting to another set that sounds the same but isn’t, Paine can make a point that probably doesn’t exist.
Hopefully both of these examples make sense and illustrate the importance of knowing what you’re talking about when you use a certain measure. There are a lot of options in the sports world, let alone the ‘real’ world, and a lot of them get at the same or similar things without actually being the same (or occasionally even similar). When you read something and the author changes his measure mid-stream, be sure to ask yourself if he’s telling the same story or what exactly has changed. And it would be good to remind authors, including myself, of the occasion dangers of having too many options.