Wednesday, January 14, 2015

Goalkeeper Stats Are (Mostly) Stupid

DC United's Bill Hamid had a great year in 2014, helping to lead a resurgent season for Ben Olsen's club.  Hamid led MLS in Save % and deservedly won the MLS Goalkeeper of the Year Award.  The other goalies receiving consideration also were near the top of the league in Save % (poor Jon Busch).

Tracing back through the recent history of the award this same trend can be seen: the "best" goalies have the highest Save %.  This seems reasonable; most of a goalie's worth to his team is measured by how good they are at stopping shots on target from reaching the back of the net (measured as Save %).  Therefore, it is intuitive that the "better" the goalie the higher their Save %.  If this is the case we would expect the best goalies to consistently outperform their counterparts year over year.  So we tested the persistence of Save % based on a sample of MLS goalies (2011-2014 seasons) who played a minimum of 2,000 minutes.  It turns out Save % is a horrible predictor, with absolutely zero relationship between Year 1 Save % and Year 2 Save %.  Breaking the stat into its components (Inside the Box Shots, Outside the Box Shots) did not help, either.

What does this mean?  For one, it confirms what most of the analytic community has known for some time: evaluating goalies statistically is really difficult.  It also, at least to me, confirms that luck/variance trumps skill when we evaluate goals scored (for strikers) or goals allowed (for goalies).

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