Sunday, 26 April 2015

Week 34: Regression Of Scoring% & Save%

Key: Scoring% = goals for/shots on target for *100 | Save%  = 100-goals against/shots on target against

Just a helpful reminder about how Scoring% and Save% behave:

Early season scoring and save percentages are all over the place after 5 games (hit the play button), things tighten a little by the 10 game mark. At the 19 game mark things are pretty tight and the spread between teams has decreased.

At 34 games the spread is this: lowest save% is 61.5%, highest 74.8%. Lowest scoring% is 22.6%, highest is 36.5%.

The spread of scoring and save percentages isn't trivial, there is a real gap between the ability of teams to score and save the shots for/against on goal but over the course of the season most teams' conversion percentages are regressing toward a mean.

Not the mean, but a mean. Some teams may well exhibit an ability to maintain high or low conversion percentages over the course of what is, if we think about it, a pretty short season (38 games). The teams who maintain high or low conversion percentages may well do so due to shot location or luck, or score effects, or talent disparity. 

The make-up of scoring or save percentages contain many different variables, part skill and part luck, but the motion chart above gives us a quick and dirty example about how scoring and save percentages regress over the course of a season.