Bobby Jenks' Missing Slider

Inspired by the discussion on whether or not to trade Bobby Jenks--a discussion I rather leave for another day, say, after the season is officially over--because of his decline in velocity and dwindling strikeout rate, I decided to take a look into the Pitch F/X data to see if we could learn anything. If I had chimed in on that discussion, which seemed to center on Jenks' fastball, I would have espoused that it was Jenks' curveball, or lack thereof, that is most responsible for his decline in strikeouts. After examining the data, however, it appears that neither the fastball nor the curveball is the missing pitch; it's his slider.

Here's another link to the data, culled from Josh Kalk's Pitch F/X tool. There are some serious caveats to apply to said data. For example, Kalk's tool only seemed to capture about 1/2 of all pitches last season (the top group in the linked table), and while we know Bobby did throw a cutter last season, Kalk's system either did not recognize them as such or chose to hide them under the greater fastball umbrella. With that in mind, you can take what you wish from the following information.

Jenks' strikes swining numbers are down almost across the board, with his fastball only missing bats on a surprisingly low 3.9% of pitches thrown, down from 7% last season. Those numbers can perhaps be explained by the bottom row of the spreadsheet which combines all types of fastball Jenks has thrown this season, resulting in a 5.8% strike swinging percentage and similar numbers to his '07 fastball in the other categories.

Jenks slider, however, has seen a precipitous drop in his strike swinging percentage that can't be explained away, down to 8.1% this year from 23.1% in '07. And while he's missing fewer bats, he's allowing more hits on balls in play (.571 BABIP in '08 vs. .150 in '07), both of which feel fluky on opposite ends of the spectrum. But, what those two numbers appear to be telling us, is that Jenks had batters off balance and weakly putting his slider in play last season, while this year they don't seem to be fooled at all.

That last thread was so lively, it was slowing down my Commodore 64.