What Fastball Velocity Value is Most Predictive?

by
November 16, 2018

Yesterday, I published an article on a few pitchers whose fastball velocity changed over the course of last season. And then my old buddy MGL showed up.

He’s right. I have so much on my plate right now, mainly my first book and a 2019 player previews, that I didn’t take it another step forward. Here are most of his answers.

First of all, I should have linked Mike Fast’s article from about 10 years ago when he first examined pitch velocity in which I re-examined a few years later. The article has held up fairly well over the years expect maybe this quote:

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From both studies, a 1 mph loss in velocity means a drop in (E)RA of .28 for starters and ~.50 for relievers. The strikeout rate drops 1% point for starters and 2% points for relievers.

For the second part of MGL’s question, he wondered on the predictive value. I ran a simple test on starters who threw at least 50 fastballs in two consecutive seasons. I used this subset as I was just trying to make sure the pitcher was a starter in each season. Moving from starter to reliever usually adds a least 1 mph to a pitcher’s average fastball velocity and I didn’t want this variable added in.

From this data, I compared the pitcher’s velocity full season, first half, second half, and September velocities in season 1 to the next full season’s velocity.

First off, a pitcher’s velocity holds up no matter which value is used.

Season 1 variable to Season 2: r-squared
Full Season: .87
First Half: .86
Second Half: .87
September: .86
80% 2H & 20% Sept: .88

It’s best to weigh the late-season values more but the difference isn’t that much.

But how about pitchers who experience the biggest drops.

I grouped all the starters who saw the velocity change by 0.75 mph (~ 1 SD above the median change) from the first half to the second half and here the correlations.

Season 1 variable to Season 2: r-squared
Full Season: .88
First Half: .87
Second Half: .89
September: .87
40% 1H & 60% 2H: .90

Second half velocity seems to be the best individual indicator as its individual value is the highest and is always part of the combined variables.

Instead of a change from the first half to the second half, here are the biggest changes from the first half to September. The value was a 1 mph changed (~ 1 SD above the median change) was used.

Season 1 variable to Season 2: r-squared
Full Season: .88
First Half: .87
Second Half: .88
September: .85
55% 1H & 45% Sept: .91

Overall, the results are the same with second-half velocity holding up the best on its own. When I start adding multi-variables, an interesting mix of first half and September numbers take precedence.

For a simple rule to cover all velocity readings, use second-half velocity values to project a starter’s velocity next season. If an owner wants to go that extra step, they can use a mix of values for those pitchers who have seen a late-season drop.





 

Jeff, one of the authors of the fantasy baseball guide,The Process, writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won four FSWA Awards including on for his Mining the News series. He's won Tout Wars three times, LABR twice, and got his first NFBC Main Event win in 2021. Follow him on Twitter @jeffwzimmerman.

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Alex ChamberlainFanGraphs Staff
7 years ago

I think there’s an issue with serial correlation here. The velocities being compared year over year at the player level are not independent, so the r-squareds are going to be artificially inflated. I don’t think it would affect the central thesis, though, so maybe much ado about nothing.

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