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.

Follow Blake on Twitter.

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1/6/2013 5:37:21 PM #

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1/9/2013 2:33:07 AM #

Joel

This evidence may not reflect home team scoring, or rather it likely doesn't ONLY affect home team scoring. There's a number of factors at play here.

There the issue of teams playing better at home than on the road. If you asked me before I read this article whether teams had higher assist rates at home or on the road, I'd have said at home, absolutely. They are more likely to run their offense, more likely to win and play, and thus more likely to have a higher assist percentage (that's not true across the board depending on offensive style, but it is relevant).

Different teams have different offensive styles that are more likely than others to lead to higher assist rates when things are going well. It seems -anecdotally- likely that teams are more likely to drift into iso heavy offense on the road than at home. I think that actually makes total sense for the 06-07 Heat in particular and even the current Heat. Plus the defenses are going to have an effect on this as well.

If the data were just a 2.7% difference, then I would be inclined to say that this is not much evidence of home team scoring, if at all. However, the team specific data makes this all more interesting.

The consistency across years, lead players and coaches would make the data all the more valuable. Also, is there a way to find out who the scorer was for each game, because that might help spot trends and explanations as well. If the Cavs still had higher assist rates in 2010-2011, when they didn't have Lebron or Kyrie's stats to puff up, that would be pretty valuable information. Then again, there's the coaching change to consider.

I think that the conclusion in this article is probably right - home team scorers have an effect on the assist rate of their home team. Nonetheless, I don't think that effect is nearly as high as some of these pieces of data might imply. I think it is much more complex than that. However, I'm willing to be persuaded.

Joel United States

1/9/2013 2:33:57 AM #

Joel

This evidence may not reflect home team scoring, or rather it likely doesn't ONLY affect home team scoring. There's a number of factors at play here.

There the issue of teams playing better at home than on the road. If you asked me before I read this article whether teams had higher assist rates at home or on the road, I'd have said at home, absolutely. They are more likely to run their offense, more likely to win and play, and thus more likely to have a higher assist percentage (that's not true across the board depending on offensive style, but it is relevant).

Different teams have different offensive styles that are more likely than others to lead to higher assist rates when things are going well. It seems -anecdotally- likely that teams are more likely to drift into iso heavy offense on the road than at home. I think that actually makes total sense for the 06-07 Heat in particular and even the current Heat. Plus the defenses are going to have an effect on this as well.

If the data were just a 2.7% difference, then I would be inclined to say that this is not much evidence of home team scoring, if at all. However, the team specific data makes this all more interesting.

The consistency across years, lead players and coaches would make the data all the more valuable. Also, is there a way to find out who the scorer was for each game, because that might help spot trends and explanations as well. If the Cavs still had higher assist rates in 2010-2011, when they didn't have Lebron or Kyrie's stats to puff up, that would be pretty valuable information. Then again, there's the coaching change to consider.

I think that the conclusion in this article is probably right - home team scorers have an effect on the assist rate of their home team. Nonetheless, I don't think that effect is nearly as high as some of these pieces of data might imply. I think it is much more complex than that. However, I'm willing to be persuaded.

Joel United States

1/9/2013 2:34:24 AM #

Joel

This evidence may not reflect home team scoring, or rather it likely doesn't ONLY affect home team scoring. There's a number of factors at play here.

There the issue of teams playing better at home than on the road. If you asked me before I read this article whether teams had higher assist rates at home or on the road, I'd have said at home, absolutely. They are more likely to run their offense, more likely to win and play, and thus more likely to have a higher assist percentage (that's not true across the board depending on offensive style, but it is relevant).

Different teams have different offensive styles that are more likely than others to lead to higher assist rates when things are going well. It seems -anecdotally- likely that teams are more likely to drift into iso heavy offense on the road than at home. I think that actually makes total sense for the 06-07 Heat in particular and even the current Heat. Plus the defenses are going to have an effect on this as well.

If the data were just a 2.7% difference, then I would be inclined to say that this is not much evidence of home team scoring, if at all. However, the team specific data makes this all more interesting.

The consistency across years, lead players and coaches would make the data all the more valuable. Also, is there a way to find out who the scorer was for each game, because that might help spot trends and explanations as well. If the Cavs still had higher assist rates in 2010-2011, when they didn't have Lebron or Kyrie's stats to puff up, that would be pretty valuable information. Then again, there's the coaching change to consider.

I think that the conclusion in this article is probably right - home team scorers have an effect on the assist rate of their home team. Nonetheless, I don't think that effect is nearly as high as some of these pieces of data might imply. I think it is much more complex than that. However, I'm willing to be persuaded.

Joel United States

1/9/2013 11:57:54 AM #

Kevin Draper

I wrote about this somewhat over at theWages of Wins site (wagesofwins.com/2012/12/14/rethinking-assists/). It will be very useful to add this metric into the analysis, but I suspect it won't change much: Pace, home/road splits, poor teammate shooting and other factors all obviously affect assist totals, but the affect is relatively small.

Kevin Draper United States

1/9/2013 11:58:08 AM #

Kevin Draper

I wrote about this somewhat over at theWages of Wins site (wagesofwins.com/2012/12/14/rethinking-assists/). It will be very useful to add this metric into the analysis, but I suspect it won't change much: Pace, home/road splits, poor teammate shooting and other factors all obviously affect assist totals, but the affect is relatively small.

Kevin Draper United States

1/9/2013 2:31:12 PM #

Bryan Grosnick

Hey guys. First of all, I'm sure Blake will be making more of the research available in the future. But there are a few things at play here.

The argument that teams will fall into higher assist rates when things are going well makes some sense. No denying that. And, by association, since many teams play better at home, a league-wide bump for assists at home makes total sense.

But as you say, it's the team-specific data that makes all the difference. Several teams have a much lower (or higher) assist rate at home from year to year ... and it's consistent in many cases! For example, the Knicks and Pacers have had a consistent home-field DISadvantage in assists since the '07-'08 season, and it's big. The Clippers have posted a HUGE home-field assist advantage within 1% point, year-to-year in five out of the last six seasons ... and that's with some serious personnel shuffling. The Nuggets had three straight seasons with a otherworldly % boost at home! And those were in the seasons when the team was in transition at the point.

I'm not sure scorers have everything to do with these numbers ... but I think that once you see the full team data, you'll be relatively surprised.

Bryan Grosnick United States

1/9/2013 2:31:28 PM #

Bryan Grosnick

Hey guys. First of all, I'm sure Blake will be making more of the research available in the future. But there are a few things at play here.

The argument that teams will fall into higher assist rates when things are going well makes some sense. No denying that. And, by association, since many teams play better at home, a league-wide bump for assists at home makes total sense.

But as you say, it's the team-specific data that makes all the difference. Several teams have a much lower (or higher) assist rate at home from year to year ... and it's consistent in many cases! For example, the Knicks and Pacers have had a consistent home-field DISadvantage in assists since the '07-'08 season, and it's big. The Clippers have posted a HUGE home-field assist advantage within 1% point, year-to-year in five out of the last six seasons ... and that's with some serious personnel shuffling. The Nuggets had three straight seasons with a otherworldly % boost at home! And those were in the seasons when the team was in transition at the point.

I'm not sure scorers have everything to do with these numbers ... but I think that once you see the full team data, you'll be relatively surprised.

Bryan Grosnick United States

1/9/2013 2:31:44 PM #

Bryan Grosnick

Hey guys. First of all, I'm sure Blake will be making more of the research available in the future. But there are a few things at play here.

The argument that teams will fall into higher assist rates when things are going well makes some sense. No denying that. And, by association, since many teams play better at home, a league-wide bump for assists at home makes total sense.

But as you say, it's the team-specific data that makes all the difference. Several teams have a much lower (or higher) assist rate at home from year to year ... and it's consistent in many cases! For example, the Knicks and Pacers have had a consistent home-field DISadvantage in assists since the '07-'08 season, and it's big. The Clippers have posted a HUGE home-field assist advantage within 1% point, year-to-year in five out of the last six seasons ... and that's with some serious personnel shuffling. The Nuggets had three straight seasons with a otherworldly % boost at home! And those were in the seasons when the team was in transition at the point.

I'm not sure scorers have everything to do with these numbers ... but I think that once you see the full team data, you'll be relatively surprised.

Bryan Grosnick United States

1/9/2013 2:32:04 PM #

Bryan Grosnick

Hey guys. First of all, I'm sure Blake will be making more of the research available in the future. But there are a few things at play here.

The argument that teams will fall into higher assist rates when things are going well makes some sense. No denying that. And, by association, since many teams play better at home, a league-wide bump for assists at home makes total sense.

But as you say, it's the team-specific data that makes all the difference. Several teams have a much lower (or higher) assist rate at home from year to year ... and it's consistent in many cases! For example, the Knicks and Pacers have had a consistent home-field DISadvantage in assists since the '07-'08 season, and it's big. The Clippers have posted a HUGE home-field assist advantage within 1% point, year-to-year in five out of the last six seasons ... and that's with some serious personnel shuffling. The Nuggets had three straight seasons with a otherworldly % boost at home! And those were in the seasons when the team was in transition at the point.

I'm not sure scorers have everything to do with these numbers ... but I think that once you see the full team data, you'll be relatively surprised.

Bryan Grosnick United States

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