There are two pitchers: One is young, with a hard sinker that he doesn’t control very well; the other is old, with a two-seam fastball he can spot pretty well though of course not perfectly.
The young pitcher throws half of his fastballs to the middle of the plate; 50 percent go down the middle and 50 percent hit the outside corner because of their tailing action. The other half of his fastballs are thrown to the outside corner; 20 percent go where they’re supposed and are strikes, while 80 percent end up out of the zone.
The old pitcher throws 70 percent of his fastballs to the outside corner; 50 percent hit the black and are strikes, while the other 50 percent miss for balls. 30 percent of his fastballs are aimed down the heart of the plate, mostly when he really needs a strike; five-out-of-six (83 percent) go for strikes down the middle as intended, while one-out-six (17 percent) miss the plate.
So in total, the young pitcher sees 25 percent of his two-seam fastballs go down the middle, 35 percent hit the outside corner, and 40 percent miss completely. The mix for the old pitcher is the same. Based on that evidence, we would conclude that the two have equal control of their fastballs. However, we know that isn’t true. The old pitcher is much better at getting his pitches to go where he wants them to than the young one. Intent is to pitching what positioning to fielding, and any conclusions we can draw from Pitch f/x about control are only as reliable as any conclusions we can draw from a play-by-play fielding system that does not know about positioning.