Correlation Co-efficients for Offensive and Defensive Efficiency

I calculated the correlation co-efficients for every NBA team’s Offensive and Defensive Points/Possession between 2009-10 and 2010-11, to figure out how much of each team’s offensive and defensive performance this season can be explained by the team’s offensive and defensive performance last season.

The Defensive correlation co-efficient I came up with was 0.6217, nearly identical to the correlation co-efficient I calculated for Defensive Win Score (0.6139).  So that was not surprising. 

The surprising result was the Offensive correlation co-efficient I came up with was a measly 0.4772.  Meaning, each team’s offensive performance last season explains only about 23% of its offensive performance this season whereas the defensive performance last season explains about 39% of team’s defensive performance this season.

Why the disparity in explanatory power?  I don’t know. I haven’t sorted that question out yet. 

Maybe defensive performance is indeed more of a group activity or leadership driven activity than offensive performance. Thus, different personnel wouldn’t make a huge difference. On the other hand, maybe the number merely reflects inconsistent shooting on an individual level.  As I said, I haven’t figured these things out.

Its interesting, nonetheless.

Footnote: The correlation co-efficient between last season’s pythagorean wins (which are a decent reflection of the team’s efficiency differential) and this season’s pythagoreans wins is only 0.4928, meaning last season’s group performance explains only 24% of this season’s group performance. Any metric whose results have a higher correlation than that can be said to do a better job of explaining the results than one gets merely looking back to last season’s performance.

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