Tuesday, April 21, 2009

Putting the "Erm..." in Sabermetrics

Particularly absurd post on Mets Geek charting each team's bullpen's average fastball speed against strikeouts. Don't even try to wrap your mind around what it might even mean to average the pitch speeds of an entire bullpen together, the results are random noise, not "a slight trend."


DU said...

The average of a group of pitchers is the average fastball speed that a batter facing random selections from that group will face.

Big problem #1: Random?
Big problem #2: Simpson's Paradox, if that's the one I'm thinking of

tps12 said...

I hadn't heard of Simpson's paradox, but yeah, it sounds like it could be an issue here.

The problem with aggregating numbers at the bullpen level is that you end up averaging away information relevant to the exact relationship (fastball speed vs. likelihood of a strikeout) you're trying to tease out.

It's like if you're looking for a relationship between vehicle size and traffic accident rates, so you start out by calculating each household's average vehicle size and how many accidents each household experiences...

Anonymous said...

Hey tps12,
This is Joe Sokolowski the guy who wrote the "absurd post." Please read the full article as I state that the chart I made shows "[it] reinforces what baseball fans already know, velocity isn’t everything." All information I used for the post was from fangraphs.com (a great website you should check out.) The point of my post was that bullpen fastball speed does not automatically mean higher strikeout rate. Thanks for linking the article and taking an interest enough to commenting on metsgeek.

tps12 said...

Even "slight trend" and "velocity isn't everything" overstate the case: the chart shows no correlation at all.

Perhaps that, and wondering whether the Rays had "figure[d] something out," was meant humorously. If so then I apologize for the misreading.

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