Thursday, January 29, 2015

One stat to rule them all? It would be a steal

It's been almost a year since Benjamin Morris wrote about The Hidden Value of the NBA Steal on, and a lot of criticism to say the least followed suit (two examples here and here).

The main criticism stemmed from the comment that "a steal is worth nine points", which caused many to throw their arms up in the air wondering how a player could all of a sudden score nine points in a single try without being in NBA Jam.

My purpose is not to review the original article, the criticisms nor review Morris's four part (!) response (kudos for tackling all the negative comments head-on). However, it is to be noted that since the steal article (Morris' third on fivethirtyeight after two others on basketball), Morris has primarily been tackling other sports than basketball (only 5 of 48, this is an advantage to writing this post so late after the fact).

Trying to take a step back, my attempt was to see how valuable indeed a steal is for measuring the value of a player. If I had to draft/trade for either a player who gets 25 points a game and 1 steal or one who has 16 points and 2 steals (to recycle Morris' example), who should I go for?

There is no perfect gold standard for summarizing a player into a single metric, although their are multiple options that get more and more sophisticated. ESPN reports RPM and WAR, defined on the site as:

  • RPM: Player's estimated on-court impact on team performance, measured in net point differential per 100 offensive and defensive possessions. RPM takes into account teammates, opponents and additional factors
  • WAR: The estimated number of team wins attributable to each player, based on RPM

So are steals are good proxy for a player's "value" assuming RPM and WAR are reliable value metrics?

I generated the following graphs linking steals per game with each of the two variables for the top 30 players in steals for the 2013-2014 season. The two graphs are extremely similar given the strong correlation between RPM and WAR.

I don't know about you, but I'm not seeing a strong correlation with steals.

This doesn't validate or invalidate Morris' analysis, but I thought it would be helpful to get some insight as to whether steals is really as powerful as the original paper would suggest.

I know I said I wouldn't comment on the back-and-forths between Morris and the critics, but one comment I had which I didn't see anywhere was around the fact that Morris seems to focus on steals per game, not my minute, not by possession. It's easier to get more steals if I play more minutes, and I might play more minutes if I'm a good player to start of with, so even if we had found a correlation it wouldn't have allowed us to reach any valuable conclusions.

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