Whenever you see basketball performance analysis where eFG% is used as the metric to measure the efficiency of FG shooting, you immediately know that the whole analysis is invalid. Since eFG% is widely used, much of the current basketball performance analysis is invalid.
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 help hide the invalidity of the underlying assumptions.
The invalidity of eFG% is due to an obvious fact: 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 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. Rather than ponder the validity the underlying assumptions, analysts concentrate on putting together new formulas or new graphics.
Why then is this important? Because currently teams’, players’ and coaches’ performances are assessed invalidly, and a lot of people – fans, owners, GM’s and even players and coaches themselves – believe in those invalid assessments. Given the invalidity of their very basic methods, performance analysts make surprisingly bold statements about how shots should be selected and how basketball should be played.
What can be done about this? Either we need to live with the shortcomings of eFG% or start using a new metric. In my Master’s thesis I suggested a metric named Shooting Quality Assessment Tool, or SQAT. Shooting Quality Assessment Tool or SQAT.
For glossary of terms, see Basketball Reference.