From Russell Carleton at Baseball Prospectus on October 24, 2017:
We need to talk about defense. Well, not really defense, but how we measure defense. OK, not really how we measure defense, but what we do with it after we measure it. A couple of weeks ago, Matt Winkelman wrote something that questioned how defensive metrics were structured, specifically around the question of how we adjust for a player’s position in putting a value on his performance.
Winkelman begins by recapping the work that’s been done previously concerning positional adjustments and how they are handled in uber-metrics like WAR(P), and then writes a paragraph that should be required reading for anyone who dips their toes in the sabermetrics pool.
The entire sample size of players for the positional adjustment study is limited to players who have played at least two different positions over the course of the study. In the original adjustments, Tom Tango tried to account for the experience difference that might come from the positional change, but what I am more interested in is what kind of players these measurements do and do not come from.
Winkelman then goes on to point out that the players who play two positions (say, shortstops who move to second base) within a season are (mostly) the shortstops who weren’t good enough to really hack it at short, and that’s why they were moved. In other words, he’s arguing that the positional adjustments on which we rely are based on a biased sample.
Read the full article here: https://www.baseballprospectus.com/news/article/34043/baseball-therapy-need-talk-defense/
Originally published: October 24, 2017. Last Updated: October 24, 2017.