Arguably the most important discovery in statistical analytics in basketball. 

A possession ends

(1) by making a field goal attempt;

(2) by missing a shot and not getting the offensive rebound;

(3) by turning the ball over;

(4) by going to the line for two or three shots and either making the last shot or not getting the rebound of a missed last shot.

Two teams in any given games are essentially limited to the same number of possessions.

Basic Possession Formula=0.96*[(Field Goal Attempts)+(Turnovers)+0.44*(Free Throw Attempts)-(Offensive Rebounds)]

More Specific Possession Formula=(Field Goal Attempts)+(Turnovers)+0.44*(Free Throw Attempts) - 1.07*(Offensive Rebounding Percentage)*(Missed Shots)