John Walsh wrote a fantastic piece on Thursday about the differences between fastballs, sliders, changeups and curveballs, and what happens when those pitches are put in play. I've done some research into this area myself and wanted to graphically present some of my findings.
One point that John made was fastballs, especially non-sinking fastballs, are hit on the ground the least often of any pitch. You can take this a step further, and look at the impact the location of a pitch has on how it is hit. The graph below looks at the percentage of each pitch type that are hit on the ground at different heights.
The most obvious thing is the huge advantage a sinker has in generating grounders compared to any other pitch. (I found sinkers the same way John did, by using all pitches with a pfx_z value of less than 6 inches). This isn't surprising, but what was a little surprising to me is how the groundball percentage of every pitch decreases at almost the same rate with increasing height. I would have thought that certain pitch types, especially curveballs, would have been much better, relative to other pitch types, when they were thrown low in the zone vs. high in the zone. I thought a curve would have a higher ratio of gb% on low pitches to gb% on high pitches than other pitch types did. This wasn't the case, so maybe the idea of a high curveball being a terrible pitch isn't totally accurate.
To get a better idea of what happens to high curveballs (and all pitch types), I looked at the slugging percentage for balls in play (including homers) based on which region of the strike-zone the pitch was thrown to. The table below shows those slugging percentages for the three vertical sections of the strike-zone. (The averages at the bottom are only for the pitches in the strike-zone and are higher than the averages in Walsh's article.)
FB SL CH CB Sinker | Avg.
Top 0.564 0.565 0.692 0.579 0.580 | 0.596
Middle 0.622 0.590 0.612 0.559 0.558 | 0.588
Bottom 0.554 0.496 0.498 0.458 0.481 | 0.497
Avg. 0.580 0.550 0.601 0.532 0.540 | 0.561
For pitches low in the strike-zone, batters have the lowest SLGBIP against curveballs, but if a curve is thrown at the top of the strike-zone, batters greatly increase their SLGBIP. Curveballs are hard pitches to hit, but the difference in SLGBIP between a low curve and a high curve is second only to the difference between a low changeup and a high changeup. Everything else being equal (speed, spin, movement, expectations of the batter, if the batter swings, etc.) a pitcher is increasing the batter's SLGBIP by roughly .100 points if he throws a curveball that isn't at the bottom of the strike-zone.
A changeup is potentially a great pitch, but changeups that aren't at the bottom of the strike-zone are hit much better than average. Low changeups are hit about as well as low sliders, but as the two pitches are elevated, the changeup gets hit much harder than the slider. A changeup above the knees is essentially a meat-ball and by throwing a changeup that isn't down in the strike-zone, the pitcher is increasing the batter's SLGBIP by at least .115 points.