The site stats would indicate we’ve picked up at least a few new readers since the close of the last basketball season. So this seems like a good time to offer a brief tutorial on how to interpret the tempo-free statistics we favor here at the Spartans Weblog.
The best place to start is the blog prereq on the old Big Ten Wonk site: This is TFS: tempo-free stats. The Wonk laid out all the basics on the history, rationale, and mechanics of tempo-free stats. Kenpom.com, the best source of tempo-free data (and now an arm of Basketball Prospectus), also has a useful glossary.
Below is my own take on tempo-free statistics, along with some numerical guideposts:
- Key question: How many times per game does a team get the ball?
- Fairly obvious explanation: The more times you get the ball, the more points you’re going to score, all things being equal.
- 2008 Division 1 median (341 teams): 67.0.
- Rule of thumb: 60 possessions/game is really slow; 75 is really fast.
Points per Possession (PPP)
- Key question: How many points does a team score per possession?
- Fairly obvious explanation: Allows for more meaningful comparisons between offensive outputs of teams that play at varying paces.
- 2008 Division 1 median: 1.02.
- Above 1.00 is generally good; below 1.00 is generally bad. This is the inherent elegance of tempo-free statistics.
- Note: An alternate calculation is “offensive efficiency,” which is just points scored per 100 possessions (i.e., 2008 Division 1 median was 102).
Turnover Percentage (TO%)
- Key question: On what percentage of possessions does a team give the ball to the other team before it’s able to create a scoring opportunity?
- Fairly obvious explanation: If you don’t get a shot off, you can’t score.
- 2008 Division 1 median: 20.8%.
- Rule of thumb: 15.0% is very good; 25.0% is very bad.
Effective Field Goal Percentage (eFG%)
- Key question: How many points does a team score per shot taken from the field?
- Fairly obvious explanation: Three pointers are better than two pointers (exactly 50% better, actually), so dividing total points scored off field goals by field goal attempts makes more sense than traditional field goal percentage, which treats 2-pointers and 3-pointers as equals.
- 2008 Division 1 median: 49.8%.
- Rule of thumb: Above 50% is good; below 50% is bad. A 3-point shooting percentage of 33.3% gets you to an eFG% of 50.0%.
- Additional commentary: Effective field goal percentage is, as a general rule, the most important of the four factors, but also (ironically) the hardest to make generalized statements about. Mathematically, it’s often helpful to break the number down into (1) 2-point field goal percentage, (2) 3-point field goal percentage, and (3) the percentage of total field goal attempts taken from 3-point range (3PA/FGA).
Free Throw Rate (FTR)
- Key question: How many points does a team score from the free throw line relative to the number of shots it takes from the field?
- Fairly obvious explanation: Points scored at the free throw line count just as much as those scored from the field.
- 2008 Division 1 median: 25.3.
- Rule of thumb: This stat can vary pretty wildly from game to game. Most teams end up in a range of 20.0 to 30.0 for a full season.
- Note: Kenpom uses free throw attempts, rather than free throws made, when calculating the free throw rate for a team’s opponents–since teams have very little control of the rate at which their opponents convert free throws. (Note added 12/15/08)
Offensive Rebounding Percentage (OffReb%)
- Key question: What percentage of offensive rebounding opportunities does a team procure?
- Fairly obvious explanation: Extending a possession with an offensive rebound creates another opportunity to score.
- 2008 Division 1 median: 32.8%.
- Rule of thumb: Numbers in the neighborhood of 40.0% are good; numbers in the neighborhood of 25.0% are bad.
The last four statistics listed above are referred to as the “four factors” and, collectively, fully explain how a team’s points-per-possession statistic has been achieved. Defensively, the same four statistics are calculated for a team’s opponents (with defensive rebounding percentage being the inverse of a team’s opponents’ offensive rebounding percentage).
Two of the factors–turnover percentage and offensive rebounding percentage–tell you how many opportunities to score a team managed to get for themselves. The other two–effective field goal percentage and free throw rate–tell you how efficiently the team turned those scoring opportunities into points.
I’m not going to do a complete explanation of individual tempo-free statistics (which aren’t quite as elegant, particularly on defense). Most of them correspond very closely with the team-level stats.
Still puzzled? Feel free to post any questions in the comments section.