Basically explained on the blog here. Sounds reasonable to me:
The most common formula for estimating possesions is (FGA – OR) + TO + (Y * FTA),
where FGA = field goal attempts, OR = offensive rebounds, TO = turnovers, Y = some number between zero and 1, and FTA = free throw attempts.
Going through the three terms in the formula, a possession can end by:
1) a shot not rebounded by the offense. An offensive rebound would continue the possession. This is captured by the term FGA-OR.
2) a turnover. (duh.)
3) Free throws – sometimes.
The only mystery here is what Y should be. First off, I’ll clear up why Y needs to be there. We don’t know how many possesions are used up by free throws, that’s why. In the ideal situation, if every trip to the line resulted in two free throws, then we could multiply free throws by one half and be done with it. However, technical fouls, the "and one" situation, missing the front end of the 1 and 1, and shooting 3 shots resulting from a 3-point attempt all deviate from the ideal situation. Oliver estimates that Y should be .4. John Hollinger in Pro Basketball Prospectus estimates it to be .44. From the data I’ve seen for college hoops, .44 is more accurate so I’ve been using that.
So that’s how possesions can be estimated, and using possesions, folks can get a better understanding of which teams do good things on offense and defense.
You might wonder why offensive rebounds are treated as continuing a possesion, rather than starting a new one. I’ve seen two good reasons. First, by including them each team’s possesions can reasonably be assumed to come out equal for each game. Second, getting and preventing offensive rebounds are skills. So if some teams do those skills better than others, it makes sense to attach those skills to a team’s offensive or defensive ability.
The most common formula for estimating possesions is (FGA – OR) + TO + (Y * FTA),
where FGA = field goal attempts, OR = offensive rebounds, TO = turnovers, Y = some number between zero and 1, and FTA = free throw attempts.
Going through the three terms in the formula, a possession can end by:
1) a shot not rebounded by the offense. An offensive rebound would continue the possession. This is captured by the term FGA-OR.
2) a turnover. (duh.)
3) Free throws – sometimes.
The only mystery here is what Y should be. First off, I’ll clear up why Y needs to be there. We don’t know how many possesions are used up by free throws, that’s why. In the ideal situation, if every trip to the line resulted in two free throws, then we could multiply free throws by one half and be done with it. However, technical fouls, the "and one" situation, missing the front end of the 1 and 1, and shooting 3 shots resulting from a 3-point attempt all deviate from the ideal situation. Oliver estimates that Y should be .4. John Hollinger in Pro Basketball Prospectus estimates it to be .44. From the data I’ve seen for college hoops, .44 is more accurate so I’ve been using that.
So that’s how possesions can be estimated, and using possesions, folks can get a better understanding of which teams do good things on offense and defense.
You might wonder why offensive rebounds are treated as continuing a possesion, rather than starting a new one. I’ve seen two good reasons. First, by including them each team’s possesions can reasonably be assumed to come out equal for each game. Second, getting and preventing offensive rebounds are skills. So if some teams do those skills better than others, it makes sense to attach those skills to a team’s offensive or defensive ability.