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.)

So…

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.

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