Introducing Arena-Adjusted Assists

by Blake Murphy 6. January 2013 16:17

When Jose Calderon was recently credit for an assist on this play (the first in the video), it had some questioning the validity of assists based on favorable home-court scoring. There’s no way, in any sense of the definition, that should have been scored an assist for Calderon.

The definition, per the NBA’s statistics manual according to this Wall Street Journal article, is as follows:

The NBA statistician's manual says an assist should be "credited to a player tossing the last pass leading directly to a field goal, only if the player scoring the goal responds by demonstrating immediate reaction to the basket."

Really, the validity question should come up for a handful of other reasons with respect to assists, but the idea of a home-advantage is an interesting one, and one that Ken Pomeroy had previously tried to tackle", albeit it was about college basketball rather than the NBA.

Question: Do scorers favor home teams when giving out assists?

According to one former Grizzlies stat-hand, it’s a lot of subjectivity and there’s room for bias from the scorer’s position. That said, that same WSJ article linked earlier had comment from the NBA that all stats are reviewed, so perhaps this is a problem that has been ironed out since the late-‘90s.

Nonetheless, the topic got me curious. In a discussion with my Beyond the Boxscore colleague Bryan Grosnick, he mentioned that he had actually pulled data on this matter before. He was kind enough to send it to me and share his findings with me.

I repeat: All of the credit for the research and pulling the stats belongs to Bryan. I greatly appreciate him sharing it with me to communicate via Hoopdata.

Bryan’s methodology for answering the question was as follows:

“I found a way to quantify it out, by comparing road and home assists to field goals made for each team, during each season from '06-'07 to '11-’12.”

That is, it compares a team’s at-home A% with its road A%. (Note that this method, using Assist Percentage, should strip out factors like pace and teams playing better at home, since it’s just A/FG. Not perfect, but it’s a start.)


Question: Do scorers favor home teams when giving out assists?

It turns out that yes, scorers tend to favor the home team when it comes to giving out assists.

Answer: Scorers give home teams a boost of 2.7 percentage points, which makes for a 4.9% boost in Assist Percentage.

In other words, the Assist Percentage for all road teams in the sample was 56.1%, while for home teams it was 58.8% (a 2.7 percentage-point or 4.9% increase).

The data that Bryan gave me contains every team season in the sample, so there is a lot of room for me to build on this initial article with further analysis. My plan over the next few weeks is to re-create the assist leaderboards in each season using what I’m roughly calling an “Arena-Adjusted Assist” metric (AAA). Those types of articles should flow from this finding and this data set fairly easily, especially given the wide disparity between teams in particular seasons.

Some Interesting Findings *The top-five most favorable home scorers belong to: Nuggets, Clippers, Hawks, Lakers, Cavaliers *The five least favorable home scorers belong to: Heat, Suns, Kings, Knicks, Grizzlies *The single highest-inflated season was the 2008-09 Nuggets with a 13% inflation *The single most-deflated season was the 2006-07 Heat with a -8.7% deflation *LeBron’s quest to average a triple-double is actually hampered by Heat scorers *Steve Nash was probably undervalued in his MVP years

Please let me know via comment/email/twitter if you have any ideas for follow-up pieces based on this information. As mentioned, I plan to do a handful of articles based off of this data set in the next couple of weeks, and suggestions are always welcome. So stay tuned for more.

And a HUGE thanks to Bryan for all his hard work.

Follow Blake on Twitter.

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Threes And Assists

by Blake Murphy 30. October 2012 22:18

One of my biggest complaints with basketball box score statistics is that I find assists to be an unfair measure of someone’s passing performance.

Growing up as a hockey player, nothing infuriated me more than making a great play and a nifty pass only to have a teammate whiff or hit the goalie in the chest, robbing me of an assist.

In basketball, it’s much the same, where a player can draw a double team and kick out to an open three-point shooter and have them throw up a brick. Or worse, they can feed a dive-man off a well-crafted pick-and-roll, only to have that player fouled, giving the player and team two points but the passer nothing in the box score.

Of course, stats shouldn’t mean much to a “team player” and many NBA teams certainly use analytics beyond just assists to evaluate passing. Even still, one tool I’ve found to be valuable here at Hoopdata is the break-down of assists based on where they set up a shooter. That is, breaking assists down by scoring range can give us some more information about how someone is helping their teammates, where assists leading to threes and shots at the rim are probably of greater value than assists leading to mid-range shots.

So I wanted to look at some players who may benefit from changes in their point guard, or for a point guard, the players around him. The following analysis is subject to some bidirectional correlation, since a point guard’s assist totals are subject to the shooting efficiency of the players he’s passing to, while the shooting efficiency of players is subject to the quality of point guard play as well.

Regardless, I wanted to look specifically at point guards who assisted on a lot of three-point field goals and players who had a lot or few of their three-point field goals assisted on. This first table shows the leaders in assists per game leading to three-point field goals, and the percentage of their total assists that the number represents.

Here we see a lot of the usual suspects in terms of assists, but a few stick out as notable. For one, Turkoglu sets up a ton of three-pointers relative to his overall assist rate, as does Jameer Nelson. This obviously has something to do with Orlando’s three-heavy offense around Dwight Howard, as they were first in the league in three-point attempts. I’d suggest you’ll see both of these players take a serious step back in their assist numbers with the loss of Dwight, three-point field goal leader Ryan Anderson, and Stan Van Gundy’s relatively unique offensive system.

This second table shows the league’s leaders in terms of the percentage of their three-point field goals that were assisted on. Surprisingly, Steve Novak just missed the cut with 97.7%, though I expected him to be at 100% since all he does is spot up.

Here we see that, even with more help than just about anyone, Gordon Hayward still struggled a bit from deep. We also see that Ryan Anderson, the league’s three-point field goal leader, may have been a by-product of a three-friendly system, something he may not benefit from on a Hornets team with Greivis Vasquez and Austin Rivers at the helm. Additionally, we see that Jared Dudley may suffer from the loss of Steve Nash, but Antawn Jamison is moving to a nice situation where he’ll be a spot-up option for the Lakers with an insane Nash and Dwight pick-and-roll combo and another great passer in Pau Gasol. He may be past his prime, but I could shoot 40% on threes on that roster this year.

This third table shows the league’s leaders in total three-point field goals, as well as the proportion assisted on.

It’s interesting to see both Deron Williams and Joe Johnson on the list, especially since Deron led the league in assists leading to threes while Johnson had a large portion of his threes assisted. Pairing them together could make both even deadlier, lending credence to projections of Brooklyn as an offensive juggernaut (defense not included). We also see that Brandon Jennings is the king of calling his own numbers for threes, showing the highest proportion of unassisted threes among the leaders.

Once again we see a Magic impact, as J.J. Reddick could lose much of his value in the new-look offense in Orlando. We also see that James Harden had a fair proportion of his threes assisted on, though it’s not extreme either way – and while Jeremy Lin wasn’t among the leaders in helping on threes, he sure found a way to get Steve Novak involved. It’s been said a lot the past few days, but the most interesting part of Harden in Houston may be how he responds to increased focus from top-tier wing defenders.

These were just a few of the leaders, with somewhat random cut-offs and analysis. Are there any other players you could see having large gains in their three-point efficiency or volume due to roster changes?

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