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July 16, 2014

Moonshot

The Matter of the Heart

by Robert Arthur


On Friday, Bill Petti wrote an interesting article at the Hardball Times on predicting the success of hitters based on the way pitchers attack them. If that sounds familiar to you, dear reader, it might be because it’s a topic about which I have written at great length in the recent past. To recap, I found that changes in the distance at which a pitcher threw to a batter could predict whether a batter was primed for a breakout or, conversely, likely to underperform his projection.

Petti found very different results in his study, however. Using a metric of his own invention called Heart%, Bill found that there was no trace of predictive power in changes in Heart% both within and across years, replicating the general approach which I had used. Let me say at the outset that I have the greatest respect for Bill and his work, especially with regards to the zone partitioning scheme which he and Jeff Zimmerman invented (which resulted in the useful statistic Edge%). So I do not mean the remainder of this post to denigrate in any way the work Bill did in this or other studies.

With that said, it’s unsettling when other analysts fail to replicate one’s results. I set out then to examine why Bill’s study didn’t reproduce my findings, and what, if anything, it can teach us about predicting batter breakouts and the strike zone in general.

It’s important to note that Bill used a different metric than I had examined. Heart% is based on dividing the strike zone into five different areas. Four of these areas are edges (up and down, left and right). The fifth is the heart, the definition of which I take from his article at THT dated May 5th, 2014:

RHH: (px >= -.43 and px <= .7) and (pz >= (1.22 + Batter Height/12 *.136) and pz <= (2.30 + Batter Height/12 *.136))

LHH: (px >= -.9 and px <= .21) and (pz >= (.65 + Batter Height/12 *.229) and pz <= (1.7 + Batter Height/12 *.229))

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<< Previous Article
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