As stated this leads to the counterintuitive result that fastballs 'move' more than curveballs.

Couldn't we counter this by moving the location of the reference pitch to the location of a fastball?

Did I ask something stupid? :p

Dave,

This sort of ties in with the question of break, and the conventional wisdom that the more a pitch breaks the 'better':

Often you hear scouts/evaluators describe pitches as having “late break” or "late movement", and I’m wondering if pitch f/x provides information on this (or if such a thing actually exists).

For example, 2 pitchers throw similar 90mph fastballs that move 5 inches into a right-handed batter — is it possible that pitcher A’s fastball starts its movement at 40′ and pitcher B’s begins at 45′ (and is therefor 'nastier' for having moved the same distance in a a shorter amount of time)? Does pitch f/x quantify this sort of thing? Or are the flight paths of pitches thrown from the same release point that have the same velocity and final movement values as they cross the plate created equal?

If I had to guess I'd say they probably are, but so many variables go into physically making a pitch, I'm curious. Any thoughts?

The topic of "break" vs. "movement" was discussed extensively two-ish years ago when pitchf/x was still quite new and people were trying to get a handle on what the data were telling us. At that time, John Walsh wrote a very nice THT article about the difference (and he even coined the two words). Here is a link:
http://www.hardballtimes.com/main/article/in-search-of-the-sinker/. Well worth reading.

Let me point out one thing that Dave did not emphasize strongly enough: "break" includes the effect of gravity but "movement" does not. Thus, a 4-seam fastball with lots of backspin has very little break (since the upward Magnus force partially cancels the downward force of gravity), but lots of movement (due to the upward Magnus force). A curveball has topspin and therefore a downward Magnus force that acts in the same direction as gravity, producing lots of break. The relative difference of break between a 4-seamer and curveball tends to agree with people's intuition.

JB, I think the break length data in Gameday refers to the distance from the plate at which the "break" value is set, the peak deviation from the reference path.

correction, break_y

Back in 2007, I put together a pitchf/x glossary and posted it here:
http://webusers.npl.illinois.edu/~a-nathan/pob/tracking.htm
Note in particular the definitions of pfx_x,pfx_z
(the "movement") and break_y,break_angle, break_length. Harry's 2nd post has it right. Mike Fast has also put together a very nice glossary:
http://fastballs.wordpress.com/2007/08/02/glossary-of-the-gameday-pitch-fields/.

JB,
Late break is a myth. It's certainly not going to be captured in pitchf/x data, where acceleration is assumed constant.

Re Alex:

I agree late break is a myth, at least in the sense that the trajectory of pitched balls are pretty much governed by the laws of physics, which tell us that the trajectory has to be pretty smooth. And certainly the constant-acceleration fit to the trajectory is necessarily smooth, as your comment indicates. However, it is also true that there is a *perception* of late break. In my view, the challenge to pitchf/x analyzers is to identify the characteristics of the smooth trajectory that give rise to the perception of late break. Josh Kalk's THT article from last year (http://www.hardballtimes.com/main/article/anatomy-of-a-pitch-slider/) is a start in the right direction. Apologies to viewers of The Book blog, where I have posted a similar comment already.

I think the break has to be adjusted for velocity. A ball lobbed to home plate with no spin would would have a very large break, like a slow-pitch softball pitch. The interesting number is the difference in the break between the actual pitch and a pitch thrown at the same speed with no spin. Part of the break of a slow curve is just the normal arc of a slowly thrown ball.

Late break is possible, theoretically. Sometimes when the ball is thrown the axis of the spin is not aligned with the direction of the ball (you want it to be perpendicular to the direction of the ball for max pressure differential). If the axis of rotation changes from being parallel to the direction of the ball to being perpendicular to the direction of the ball, while it the ball is on its way to home plate, the ball can curve more sharply towards the end of its flight to home plate. An example of a pitch whose axis of rotation is parallel to its direction is the gyro-ball or fake slider. But the axis of rotation can change relative to the balls direction, during its flight. A gyro-ball can turn into a true slider as the axis of rotation becomes more perpendicular to the path of the ball as it nears home plate.