# Nylon Calculus: Week 2 in Review — Free boards

Two weeks of basketball are already gone, and what was once perceived as a “boring” season where the Golden State Warriors were destined to steamroll the league and meet the Cleveland Cavaliers in the finals has turned into one with a more level playing field with a handful of genuine surprises, from Joel “the Process” Embiid living up to legend to the Minnesota Timberwolves floundering under their first season with Tom Thibodeau.

There’s also some sort of election in the US, and NBA fans will be distracting themselves with games to ease the anxiety here in the States and abroad.

## Russell Westbrook and BPM

For those who follow advanced stats, Russell Westbrook’s campaign of destruction is quite entertaining for a curious reason: he appears to be “breaking” certain metrics. While over any short subset of games you can have some outrageous values, there’s still an interesting question buried here: is there something about Westbrook’s change that could inherently flummox, say, BPM, Basketball-Reference’s box-score metric?

You can see the details on the BPM calculation here, but it doesn’t give you an intuitive sense of how important the various weights are in measuring Westbrook; there are too many interactions. Usage rate is the likely culprit here, since he’s on track to record the unofficial record for BPM. I’ve prepared a chart below varying usage rate while holding his other values constant. This is to show the elasticity of BPM in regards to usage rate. I’ve also included three extra cases with different values of AST%, TRB%, and TS%. Note that those lines don’t show you how BPM changes in regards to those variables; the lines show you how usage rate changes with different baselines. And yes, it appears Westbrook’s monstrous usage is the source of much of his BPM power, but it’s greatly increased with a high AST%.

There’s one important detail I left out for the calculations: AST% is an estimation of teammate field goals assisted, so when your usage rate increases your AST% does too, even if you don’t tally more assists. I used a quick adjustment factor for the chart above, and without it there are significant changes. Westbrook’s BPM is driven by historically high levels of USG% and AST%, and in conjunction those two give a lethally high output.

## Boomerang assists

With SportVU data, there’s been an explosion of new stats, and the new stat list is even longer with a few clever combinations. For example, with a secondary or “hockey” assist, a player can actually assist himself. After this conversation here, I’m pretty settled on the name of boomerang for that type of assist. And it’s actually a pretty neat demonstration of player value: for the shooter, assisted field goals are generally not “worth” as much as unassisted field goals in the sense that the passer gets some of the credit. But if you set up the passer, you get some of the value back through the secondary assist. The relative value of hockey assists to old world assists is up for debate, however, but you can see how that value can come into play.

## Bullish market: Chicago’s start

The Bulls, after suffering through a few weeks of criticism for their roster design, surprised the league by opening 3-0 with a flurry of made 3-pointers. They’ve regressed, having lost three in a row, but people are now more optimistic about their chances this season and the team itself is eager to prove doubters wrong. But let’s take a step back and think about their projection and the long-term plans here.

First of all, Dwyane Wade’s 3-point percentage is unsustainable, to put it lightly. Before this season, his career 3PT% was 28%, and so far he’s shooting 48%. Even if one concedes that, after only four games and an impressive post-season, his 3-point shooting is significantly different now, his percentage will drop. This is actually one of their most important gauges for the season because if Wade is a reliable outside shooter, it opens up the floor for the rest of the team.

However, what we’re looking at is one of the best-case scenarios for Chicago — their new veteran guards are playing better together than expected, and Wade has been shooting well. But, long-term, what are the benefits here? What will the future Bulls team look like? Loading up on Rajon Rondo and Wade is a short-term ploy, and it’s not enough by any reasonable standard to make them a contender. It appears like it’s one of those weak short-term moves where the big names impress a broad section of the fanbase and some people in the front office without a major overhaul.

We can get excited about the Chicago offense performing well despite having the spacing properties of a singularity, but the defense is clunky and the offense has already sputtered, even against poor defensive teams like Indiana. And I’m not sure how this helps form the Bulls team of the future, because this was supposed to be Jimmy Butler’s team, with Derrick Rose gone, and neither Wade nor Rondo are decent building blocks at this point in their respective careers. It’s strange because they just tore down the old core of the team — Joakim Noah and Derrick Rose — to install this one. What’s the plan?

## The steal-block

Every once in a while, you’ll see an especially potent block where the ball isn’t swatted away; it’s basically caught and the defender controls the possession change entirely. Myles Turner, as seen here against Tarik Black, had one last week using two hands, and ESPN’s NBA Twitter wondered if it should be counted as a block or steal. I won’t assume the question was rhetorical because many people actually do have the same thought. If the action is considered a shot attempt, then it’s a block — it’s as simple as that. A steal can’t occur with a shot attempt because it’s a turnover. It was a great week for these types of plays, however, because we got another great example from Hassan Whiteside below, who needs only one hand to block Toronto’s Pascal Siakam in the first quarter. By the way, you actually get credited with a block and a rebound in the play-by-play.

## Rise, Lakers, rise

The Lakers, who have appeared on way too many national games while they were mediocre to terrible, are actually fun again — they’re 4-3 with a positive point differential. Their pace is up by several points, their assist rate is up, Julius Randle is playing exceedingly well, and Larry Nance Jr. is dunking like there’s a hidden trampoline on the court. This is also a good example of where coaching matters because Byron Scott was arguably the least respected coach in the league while Luke Walton was coming from a record winning organization. All in all, it’s nice to see a historic franchise look so fun on the court, and it might have something to do with the weight of Kobe Bryant lifted from the court.

## Weekly Golden State theorizing

The Warriors have largely been disappointing, since they’re following a season where they won 73 games and replaced a mediocre starter with Kevin Durant. Based on the evidence we see just from their play this season, they don’t look like a team that will lead the league in wins, and maybe not even in their own division. Of course, based on how good everyone was in prior seasons I’d assume things will improve quickly, but there are legitimate reasons to be concerned.

Firstly, as I’ve pointed out before, the Warriors were lucky in opponent three-point percentage last year. Plus, if their offense isn’t performing at historic levels, they’re setting up their opponents with easier opportunities after the rebound. And I really think these problems would have occurred without the major changes too. My biggest fear for the team before Durant was that their defense relied heavily on Andre Iguodala and Andrew Bogut being elite defenders, a pair of players who, due to a combination of age and health, could decline rather quickly without much warning. In essence, that’s what has happened now. Zaza Pachulia looks like he’s lost a step, which is what I thought would happen with Bogut.

We can’t know for certain what’s going on or we won’t know how good they truly are for the rest of the season until the games are actually played, but if their defense slips considerably then the issues become compounded and leaks spring everywhere. Without stops on defense, there are fewer fast breaks; with fewer fast break finishes, opponents can start their own fast breaks more easily; etc. This dovetails into chaos theory, where small changes create large effects, all because a few parameters are different this season.

## Untangling rebounds: Free throws versus field goals

While rebounds are a basic statistic that we all consume thoroughly and use to judge players, there’s a basic classification error that’s usually ignored: not all rebounds are the same, and ones from free throws are definitely different. Presumably, grabbing a defensive free throw rebound is not as valuable as a defensive one, and many players do not even get a fair opportunity to snatch one from a free throw because they’re not near the basket.

Separating free throw rebounds from field goal rebounds isn’t tough though — you can separate them using play-by-play data pretty easily. It’s just rarely been used, but we’re coming up on two decades of this data; let’s start using this. By the way, one popular stat does separate rebounds: ESPN’s RPM. There is a measurable reason for doing so. Since free throw defensive rebounds are easier to grab, for instance, a player can get an inflated DRB% if his team had an unusually high number of opponent free throws, or if said player sniped those easy boards from teammates regularly.

Starting with the rarest action, the player with the highest number of offensive rebounds from free throws is Mehmet Okur, who I’m sure wouldn’t be the first, or 20th, player anyone would have guessed. For the uninitiated, Okur was a three-point shooting Turkish center known for his stint on the Utah Jazz. He was never known for his rebounding, and his total offensive rebounding numbers were usually weak, mostly because he hung out at the perimeter. But he was great at going for free throw rebounds, and the Jazz had some of the highest opponent free throw rates in NBA history under Jerry Sloan. Note that Okur’s ratio of free throw offensive boards to total offensive boards was quite high compared to everyone else on the top 15 list.

Table: Free throw ORB leaders by season, 1997-2016

 Player Season FT ORB/ ORB FT ORB Mehmet Okur 2006 0.11 24 Carlos Boozer 2007 0.09 20 Mehmet Okur 2007 0.12 19 P.J. Brown 2002 0.07 19 Danny Fortson 2002 0.07 19 Elden Campbell 1997 0.09 18 Tim Duncan 2000 0.07 18 Jayson Williams 1998 0.04 18 Rasho Nesterovic 2004 0.07 17 Tim Duncan 2003 0.07 17 Greg Monroe 2014 0.06 16 DeMarcus Cousins 2012 0.06 16 DeMarcus Cousins 2011 0.07 16 Tim Duncan 2010 0.07 16 Mehmet Okur 2008 0.12 16

Moving onto defensive free throw rebounds, Antoine Walker, another three-point shooting big man, is the clear winner here. His defensive rebounding numbers overall were decent for a power forward but nothing special, and none of his other seasons ranked high, so perhaps it’s just a blip in the data. You can see the top of the list is dominated big all-stars. And, please note, there’s nothing wrong with grabbing an easier rebound here; you still want to prevent the offense from recovering the miss. It’s just not as valuable as a field goal rebound (usually.)

Table: Free throw DRB leaders by season, 1997-2016

 Player Season FT DRB/ DRB FT DRB Antoine Walker 1998 0.24 108 Marcus Camby 2008 0.14 102 Kurt Thomas 2005 0.18 101 Kevin Garnett 2000 0.15 97 Patrick Ewing 1997 0.17 95 Dirk Nowitzki 2003 0.15 95 Carlos Boozer 2007 0.17 94 Dwight Howard 2006 0.15 94 Dwight Howard 2008 0.12 93 Karl Malone 1997 0.18 92 Dirk Nowitzki 2005 0.16 92 Karl Malone 2000 0.18 92 Carlos Boozer 2008 0.16 90 Loy Vaught 1997 0.18 90 Olden Polynice 1997 0.22 89

If you just go by proportion of defensive rebounds that are from free throws, there are actually a lot of wing players who dominate, like Jalen Rose and Richard Jefferson. That may seem odd, but some front lines will box out for the third defensive player along the blocks. You can also see the variation of the proportion of these rebound types in the two graphs below. There are vast differences among some seasons in terms of where the rebound originated.

Lastly, let’s look at how the proportion of free throw rebounds has changed over the past two decades. Via the graph below, you can see the share of these rebounds grabbed by the offense change year-over-year. There were a few blips, notably the 1999 lockout season, but there’s been a significant downward trend in free throw rebounds — defenses are grabbing more and more of them. After a couple seasons near 0.15, the share is down to about 0.10. That’s a large decline.

What’s interesting here is that total offensive rebounds have been going down for a long time too, and many people theorize this is because of 3-point shooting and defensive tactics, wherein teams want to stop fast breaks instead of crashing the boards. But free throws are a basketball play where the environment is completely different, and 3-pointers definitely can’t influence the results here, yet the rate is still declining. There’s something more going on, and given how offense/defense are tangled — it’s tough to tell if a change is due to better defense or worse offense — it’s hard to find a satisfactory answer.

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If I had to speculate, I’d base it on how, generally over time, a sport improves and free throw rebounding on defense is about mitigating mistakes. You have the advantage of position and an extra player near the rim, so defenses should grab a huge percentage of the boards. But I’m not quite sure, and I didn’t expect this either. But surprises like these are why we analyze data.