When comparing the total number of possessions, we find…

For seasons 2002-06, NBA teams average roughly 93.1 possessions per game.

For the 2014-15 season, NBA teams averaged roughly 95.7 possessions per game!Is this change significant?

**A t-test suggests that it is *highly* significant (p<<0.01): that we are seeing significantly more possessions per game nowadays than in the mid-2000’s.**What’s causing this change?We fit a linear model for possessions in the same way as Kubatko et al. (2007), i.e. by ordinary least squares (OLS), making a formula for possessions in terms of seven box score stats: FGA, FG missed, FTA, FT missed, OREB, opponent DREB, TO.The Kubatko 2007 model was…Fitting with the 2014-15 data, we get the following model:Right off the bat, we observe that all coefficients have the same sign and relatively similar values. Moreover comparing model correlations with actual possession numbers from 2014-15 shows that for Kubatko (2007) R^2 = 0.9483 while for the updated formula (2015) we have R^2 = 0.9496. So the old and new formulas have similar accuracy.

However the formulas aren’t identical. The constant term is higher in the 2014-15 model, consistent with the idea that there are now more possessions per game. Also, moving from mid-2000’s to now there is a decrease in the contribution of OREB: Namely, there are more offensive rebounds per possession now than ten years ago. To see this inverse relationship, imagine two games, one in each decade, and suppose both games have 100 possessions and identical box score stats — except for offensive rebounds. Solving for the number of rebounds, we see it is inversely proportional to the regression coefficient. A smaller coefficient therefore suggests there are more offensive rebounds per possession now than in the mid-2000’s. While the change is not quite significant at a confidence level of 95%, it suggests a trend that may become significant with more observations.In upcoming posts, we will continue breaking down possessions and discussing the methods used in NBA analytics. We hope that those who have interest in collaborating offer feedback and consider writing with us.

*[post script] Brain & Basketball believes in reproducible science and open source software. Let us know how we can help and provide for your own analysis.*