Posted by Neil Paine on December 23, 2010
You'd better read this post from August if you haven't yet.
In it, I looked at the year-to-year correlation coefficients for various offensive rate stats (TS%, AST Rate, TOV Rate, FTA/FGA, & OReb%) when a player changed his role in the offense. Essentially, I concluded that offensive rebounding and assists are relatively immune to changes in a player's possession usage, foul-drawing & turnover avoidance are less immune, and scoring efficiency is the most prone to fluctuate with a role change.
Today I wanted to expand on that post by adding another variable into the mix: changing teams. Other than the new variable, though, this study's format is basically the same as in the first post -- except I used touches per minute rather than possession % to define a player's role, and I added Dean Oliver's Offensive Rating (ORtg) into the mix.
Here's the setup: Once again, I found every player from 1974-2010 who was between 24 and 34 years old and played at least 1,000 minutes in back-to-back seasons. I then sorted those players by the absolute change in their touches/min, and took the top quartile as my sample of players who changed roles. I also isolated players who played for a different team than they had the previous season, forming four groups: players who changed team & role; players who changed role only; players who changed team only; and players who changed neither team nor role. Finally, I ran correlation coefficients on the year-to-year offensive rate stat performances for each group:
|Changed Team + Role||414||0.617||0.593||0.792||0.715||0.742||0.924|
|Changed Role Only||717||0.695||0.686||0.875||0.756||0.828||0.943|
|Changed Team Only||779||0.563||0.556||0.963||0.735||0.811||0.930|
|Didn't Change Team or Role||2611||0.719||0.706||0.974||0.802||0.842||0.944|
- Shooting/offensive efficiency is actually far more impacted when a player changes teams than when he changes roles. This suggests that a team's system, coaching effects, and teammate interactions play a much bigger role in determining shooting percentages than "skill curve" effects.
- Assists are more dependent on role than team -- for obvious reasons, how much you have the ball in your hands is a major factor when it comes to how often you set up your teammates.
- Turnovers, fouls drawn, and offensive rebounds are all more team-dependent than role-dependent, but the differences are nowhere near as drastic as those observed in shooting efficiency or assists.
- For players with no major contextual changes, assists and offensive rebounds are relatively stable; foul-drawing and turnover avoidance are less consistent; and scoring efficiency is the least consistent of all. This mirrors the findings of the original study.
- Finally, it bears mentioning that even at its least consistent (TS% for players who changed teams), NBA player performance is way more predictable than that of the NFL quarterbacks Jason Lisk looked at in the football study that inspired these posts.