How goalies age, and the survivorship bias problem

Eric T.
December 06 2012 02:10PM

Brodeur
Older goalies can still get the job done, but sometimes they need to sit down and rest for a bit.
Photo by rubyswoon via Flickr

This morning, Steve Burtch published a study arguing that goalies do not show any appreciable decline with age. The key plot was this one, in which the x-axis is the goalie's age and the y-axis is how many standard deviations above or below league average he was (for goalies with at least 30 games played):

Goalie vs age

It's clearly true that the observed performance of old goalies isn't appreciably worse than the observed performance of young goalies. The problem is that word "observed" -- we don't get to observe the performance of all goalies at all ages. JaredL previously looked at this issue and showed how few goalies continue to play heavy minutes into their late 30's. This creates what is called a survivorship bias.

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