Introducing the Kirby Index: A New Way to Quantify Command

Release angles!

No Pitch Plots today but I do have a new post about quantifying command using release angles that went live on FanGraphs this morning. Here’s the basic idea:

Release trajectories… reliably provide information about pitcher intent like no other variable in the public sphere. In theory, pitchers who have better command should have trajectories — vertical and horizontal release angle pairs — that cluster tightly in specific locations. My statistic — the Kirby Index, named for Mariners starting pitcher George Kirby — will be the first public metric that tests this theory.

I got pretty excited about this about a month ago when I realized release angles were this powerful variable that explained a lot about pitch locations and nobody besides Alex Chamberlain was really talking about them. So I created the Kirby Index, which does a (very basic) job of assessing how good a pitcher is at throwing his four-seam fastball to the exact same location. It turns out that the simple Kirby Index is “stickier” year-to-year than two models (Location+ and PitchingBot) that also attempt to quantify command.

The 2024 Kirby Index to date is linked in a Google Sheet here — I’ll plan to update that Google Sheet every week or so with updated rankings. (For more frequent updates, you can create your own Kirby Index at home using my code.)

I think I started reading Dave Cameron at U.S.S. Mariner when I was in middle school; when he started writing for FanGraphs, I read him there as well. By the time Jeff Sullivan left Lookout Landing for FanGraphs in 2012, I was reading the site frequently, and have continued to do so. Seeing my byline on the same site that published my two favorite baseball writers growing up is pretty fun. I hope you read and enjoy the piece — I’ll be posting a new Pitch Plots about Shota Imanaga on Monday.