Monday, August 01, 2005
Stats Explained
Let’s start with the most basic stats to measure the ability of a team’s offense and defense.
Offensive efficiency
Points scored per 100 offensive possessions.
Defensive efficiency
Points allowed per 100 defensive possessions.
In order to compute efficiency, we need to know how to compute possessions.
Possessions
We can estimate possessions very well from box score stats by using this formula.
FGA-OR+TO+0.475xFTA
For each team, possessions are counted for the team and their opponents, and then averaged.
Efficiency gives us broad view of how well the offense or defense functions, but we can break efficiency into what Dean Oliver dubbed the Four Factors. Shooting, rebounding, turnovers, and free throws provide the basic components of efficiency.
Effective field goal percentage (eFG%)
(FGM + 0.5*3PM) / FGA
Shooting is measured by effective field goal percentage, which differs from conventional field goal percentage by taking into account the extra value of a made 3-pointer.
Offensive rebounding percentage
OR / (OR + DR)
Defensive rebounding percentage can also be computed, using defensive rebounds in the numerator.
Turnover percentage
TO / Possessions
Free throw rate
This can either be FTM/FGA or FTA/FGA. Typically, for team offense FTM/FGA is used, while on defense FTA/FGA is used.
There are other team stats that are less important than the Four Factors, with the common approach of converting the standard per-game stats to per-opportunity.
Assist Rate
A / FGM
Block Rate
Blocked shots / Opp. 2PA
Steal Rate
Steals / Defensive possessions
All of the above stats can apply to individuals in some form, also. There are two other stats that are applied to individuals that aren’t applied to teams. These stats were developed by Dean Oliver, and the formulas are far too complicated to list here. His book, Basketball on Paper, is worth buying if you are interested in how the calculations are performed.
Offensive Rating
This is the personal version of team offensive efficiency.
Usage (% of possessions used)
This describes a player’s role in the offense, by explaining how many of his team’s possessions a player is personally responsible for ending while he is on the floor.
A simpler version of personal efficiency is this one
Points per weighted shot (PPWS)
Points scored / (FGA + 0.475*FTA)