Often you see basketball analysis where eFG% is used to measure the efficiency of FG shooting. Unfortuately, this makes the whole analysis susceptible.
Nothing helps: not the huge amount of the data, not the sophistication of the analysis methods, not the great looks of the graphics. In a way, they only make matters worse since they may hide the invalidity of the underlying assumptions.
The invalidity of eFG% is due to this: eFG% does not consider all FG shots. The equation of eFG% is: eFG% = (FGM + 0.5 x 3PM) / FGA. The problems is that not all FG shots count as FGA.
If the shooter gets fouled in the act of shooting and misses the shot, that FG shot is not considered an FGA in the statistics. Consequently, eFG% does not measure the efficiency of all FG shots but the sample is systematically biased. This makes all performance analysis, where eFG% involved, invalid when it comes to measuring the efficiency of FG shooting.
Despite this obvious bias, eFG% continues to be widely used to measure the efficiency of FG shooting. Why? Probably because it is easily derived from the basic box score stats. Thus implementing eFG% requires no extra work.
Why then is this important? Because currently teams’ and players’ performances are assessed invalidly. Yet a lot of people – fans, owners, GM’s and even players and coaches themselves – believe in those assessments. Analysis based on eFG% is used to argue how shots should be selected and how basketball should be played.
For glossary of terms, see Basketball Reference.