If you average only 5 mph separation, there is variance around that. Say a standard deviation is 2 mph, you're going to be at 3 mph or below quite often, and getting hit hard. If you average 8 mph separation, then even w/ variation you're almost always going to be in your sweet spot with your changeup.

Good point Joel, but continue this out. Hamels and Santana who have very good changeups are at about 11% slower. With variation they should at times throw ones 13 to 15% slower which will get hit hard too.

At the low end, perhaps the absolute speed of the change matters as well as the differential (or instead of the differential). If the pitch is slow enough, hitters can adjust and whack it. So, a guy with a 95-mph fastball can do well at -12% (84 mph), but an 89-mph FB pitcher might get killed with a -12% change (78 mph).

Guy,

That is a great point. It would be interesting to do the analysis looking at how absolute changeup speed and the speed difference from the preceding fastball jointly explain changeup success. I bet you something like what you suggest is going on.

these are interesting results, but of course there's also a question of how well a pitcher hides his changeup in his delivery. while some of that is connected to the speed of the pitch, a lot of it is separate.

Error bars! Woohoo!

Besides the methodology, I assume there are at least two differences between your data and Josh's - please correct me if I'm wrong.
1) Josh adjusted data for park/climate factors
2) Josh applied his own pitch classifications

Good point Harry, I have done neither.

I swear, like two days ago I was just thinking about whether somebody had researched this. Excellent work!

I'd love to see the stats for former closer and changeup master Doug Jones, since his fastball wasn't much better than 80.

There are a few cases that may violate the rule of optimal separation. A lot of it depends on how you measure changeup success -- do changeups 'succeed' only based on the results from the changeup itself, or also by how the hitter reacts to later fastballs. A really good changeup should make it more difficult for the batter to time the next pitch as well.

I took a look at some pitchers with great changeups in 2008, and Clay Buchholz's 75-80 changeup (which obtains excellent results by itself) doesn't necessarily make his 91-95 fastball better:

http://sonsofsamhorn.net/index.php?s=&showtopic=34118&view=findpost&p=1724531

Al,

Good question. I used the pitch f/x data to do the analysis which only goes back to 2007. You need pitch-by-pitch data to do it. I think BIS has pitch-by-pitch data going back further, but I am not sure how far.

Alex,

Good point, there are definitely exceptions. These are averages and I am sure there are pitchers who can succeed with less than 5% and greater than 12% separation. It would be interesting, as you imply, to see the impact that changeups have on fastballs later in the at-bat.