More about Joba's slider

Back in October, I wrote about how Joba Chamberlain appeared to be losing command of his slider. In this post, I would like to return to that topic.

I watched most of Chamberlain's outings last year, and I noticed that his slider looked much different at the end of the season than it did previously. What I thought saw was increased velocity, along with less movement and more "hanging pitches."

By looking at the above graph, which shows the average slider velocity in each of Chamberlain's 31 starts, it's clear that something happened to his velocity at the beginning of September. Using this as a guide, I decided to split up Joba's sliders into a September and pre-September set (unfortunately creating a much smaller sample size for one of the groups). Honestly, I expected to see a huge discrepancy in terms of results, but this was not the case at all:

	Avg. Speed	pfx_x	pfx_z
4/09-8/09	84.22	2.06	-0.70
9/09	86.16	1.14	1.71

Pitch#	Swing%	Whiff%	Wide Zone%	Chase%	Watch%	RSv/100
509	48.3%	41.1%	51.5%	35.8%	39.7%	1.37
89	53.9%	45.8%	52.8%	41.7%	40.4%	1.41

The rates are extremely similar. He actually got more swings and misses on the slider in September, which I would not have expected. The pure stuff looks different, though. In September, the pitch was on average 2 mph faster than it was the rest of the year; it also cut an inch less horizontally and lost two inches of vertical drop. Maybe this is all just PITCHf/x variance? To try to get an answer, I did a quick look at the speed and movement of Chamberlain's fastballs from the same period.

	Avg. Speed	pfx_x	pfx_z
4/09-8/09	92.52	-3.88	9.91
9/09	92.55	-5.32	10.27

So yes, it is pretty clear that at least the velocity and vertical movement underwent a transformation at some point. Still, this doesn't tell us anything about the hanging pitches. Maybe this pitch height frequency chart can answer some questions.

The green spike above the middle of the strikezone is clear to see, and it would indicate that Chamberlain was hanging a greater percentage of sliders in the upper half of the zone in September than earlier in the year. Even though he got similar results with his slider in September, as I showed above, it's certainly not a good thing to leave too many sliders up , which he was doing to an extreme in the postseason.

Gameday PITCHf/x data is from MLB Advanced Media; it can be easily accessed via this tool.