Did Arnberg's Firing Drop Bud Norris' Strikeout Rate?

HOUSTON - SEPTEMBER 14: Pitcher Bud Norris #20 of the Houston Astros throws against the Philadelphia Phillies at Minute Maid Park on September 14, 2011 in Houston, Texas. (Photo by Bob Levey/Getty Images)

After Bud Norris' excellent start Wednesday against the Phillies, I considered a question about his season. Was Bud affected by Brad Arnsberg's firing? 

Certainly, his season started out well before training off some in July. Just taking his first two month splits and comparing them to July and August show a slightly different pitcher, with lowered strikeout rates but similar peripherals.

Was there a reason Bud was getting more strikeouts with Arnsberg, who was fired on June 15? Let's look at the Pitch F/X data for Norris from before and after that date.

Before Arnsberg:

Type Count Selection Strike Swing Whiff Foul In Play
FF 723 50.6% 61.0% 41.8% 5.5% 19.4% 16.9%
SL 564 39.5% 68.1% 55.1% 20.7% 18.8% 15.6%
CH 141 9.9% 53.9% 41.1% 9.2% 12.8% 19.1%

After Arnsberg:

Type Count Selection Strike Swing Whiff Foul In Play
FF 793 49.3% 61.7% 41.5% 5.3% 18.2% 18.0%
SL 599 37.2% 65.4% 53.8% 19.0% 17.0% 17.7%
CH 217 13.5% 62.2% 47.9% 7.4% 20.7% 19.8%

 

The first thing you'll notice is a pretty even split on number of pitches thrown between the two halves. You'll also notice that there's not much difference between the two charts. Norris throws three pitches and leads with his fastball. His slider is his best pitch and that usage rate remained unchanged wit slightly lower strike rates and swing rates. 

The only real, substantial difference is an uptick of about 3.5 percent in the changeup usage rate. This is where the drop in his strikeout rate can be attributed. 

See, Norris is getting about 10 percent more strikes with the change than before and saw an increase in his swing percentage on the pitch as well. Since his whiff rate dropped slightly since the beginning of the season, that means more hitters were making contact with the change. 

However, they weren't putting the change in play and were instead fouling it off. If you follow the percentage increase in the strike, swing and foul ball percentages, they're all about the same. So, all those extra changes Bud is throwing are getting fouled off, which leads him to use his slider or fastball as an out pitch and may be lowering his strikeout rate.

It seems counter-intuitive that if Norris is throwing his change for more strikes, his strikeout rate will lower, but that's the best explanation I can see from some rather incomplete data. I'll also add the disclaimer about looking at partial season data for pitches. We have a pretty big sampling here, but it's still not a full season's worth of data. 

If I had to guess, I'd say this is more about Norris learning how to command the changeup. It could be his best pitch, but he's throwing it where hitters can get to it right now, even if they're just fouling it off. If he can learn to sequence it better and command it more effectively in the strike zone, Norris' strikeout rate could jump again.

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