Sarris: Breakout sluggers, predicted by fastballs

From SABR member Eno Sarris at on January 26, 2015:

You can tell a lot about a hitter by how many fastballs pitchers are willing to throw him. The bigger the bat, the more likely it is to see junk. Turns out, small changes in the number of fastballs a hitter sees can help us project that hitter better. 

Sort the leaderboard for lowest fastball percentage, and you’ll see it immediately. It’s full of sluggers at the top. Reverse the filter and it’s mostly slappy speedsters. Rob Arthur took a more scientific approach and showed that isolated slugging and fastball percentage are indeed correlated negatively — sluggers see fewer fastballs

Rookies see more fastballs when they come into the league. Over the past five years, the league saw 57.5% fastballs, and rookies saw 58% fastballs. That’s not a large difference, but it comes in a large sample. Then again, it’s not a large difference, period. Over the course of a season, a rookie with 600 plate appearances would be expected to see 12 or so extra fastballs. 

In any case, even if this effect is small when you zoom out, it seems that individual differences in fastball percentage are predictive of future strong work. As Arthur said when he did the gory math behind this statement, “Fastball frequency normally varies according to the pop of the batter, so that when it changes, it may be indicating a change in the underlying skill level of the same batter.”

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Originally published: January 26, 2015. Last Updated: January 26, 2015.