Many believe, as do I, that assists should take on a larger role in basketball analysis, specifically in player evaluations. Whether it be in Wins Produced, PER, or even my HDR, I think assists can be a great way to control/test for certain things (shot creation, systems, inherent value). However, I have recently begun to shift my position, as the limitations of assists have become quite clear to me. Here are a few quick reasons why it is difficult to use percentage of shots assisted, or assists, in a meaningful way.
1.) They don't have a high correlation to shooting percentage. I regressed percentage of assisted shots and effective field goal percentage for every player from 2006-2012. I then broke this down by position, and determined correlation with an R2 value.
PG - 5.6%
SG - 3.6%
SF - 5.9%
PF - 5.6%
C - 10.4%
On the surface their IS a correlation, especially for centers, but not a huge one. Of course we could take this further, by controlling for players changing teams, system quality, and coaching, but I would still like to have seen a stronger initial correlation.
2.) They aren't assigned consistently.
The assist percentage for an average NBA team is around 57%. Up to 20% of that 57% can vary based on the arena you are playing in, the scorekeepers involved, and random chance (Hoopdata). That is a huge blow to the credibility of an assist.
3.) They aren't a complete stat.
Assists attempt to measure how much a player helps his team, and how much a team/player is being helped. Unfortunately, its difficult to do this when assists are only recorded for made baskets. To truly delve into the concepts of shot creation, teammate elevation, and system prowess, will require the use of potential assists. We need to know how many times players are set up/set someone up for a good shot, not just how many times a player makes the basket. Imagine trying to judge scoring prowess using field goals made, without access to field goals attempted.
"You want to use the data because it gets at something important but it’s so subjective that it’s difficult" - Dean Oliver
Dean said it best. The concept of an assist is integral to the understanding of basketball. Its important to know who gets set up for open shots, who is setting those people up, and how this effects the team on a league wide basis.
Unfortunately, due to the reasons I listed above, the traditional assist metric captures only a fraction of this important process.
To recap, the assist is a crappy metric. Not because its measuring something that doesn't matter, but because its hardly measuring anything. For those of us who want to determine just how much certain players are helped/helping, it seems we'll have to be patient for a little while longer, because the data is just not there for serious analysis.