I had some comments/requests for additional context about the charts I showed last week and other aspects of my linear weights articles, so I wanted to present those and clear up some confusion about the charts from last week.
Among others, Richard Aronson commented here last week about my statement that left-handed hitters liked the ball down and in, but mentioned that the linear weights in those areas were still negative. He suggested that I break up the charts by balls in play and balls not in play and see if the statement still held true. The chart below shows how left-handed hitters fared against all pitch types in any count, but only when they swung at the pitch.
The chart shows that pitches in the middle of the strike-zone, both horizontally and vertically, benefit the hitter, while pitches on the corners, especially the lower ones, favor the pitcher. In addition to only looking at swings, this chart differs from the one I presented last week in that it looks at all pitch types, not just fast balls. Maybe left-handed hitters are able to hit down and in fastballs very well. We can test that and...
crap. They still can't hit pitches in that location very well, and its interesting to see that they are able to hit fastballs on the outside half of the plate much better than they can hit fastballs on the inside. Generally inside fastballs are thought of as places where a pitcher can get hurt, while outside fastballs are encouraged. One reason left-handed batters are able to hit outside fastballs better than inside fastballs could be because of the extra fraction of a second an outside pitch affords the batter. An outside pitch is hit slightly after it crosses the plate, and giving the batter an extra 'beat' to track the ball. In order to be driven, inside fastballs need to be hit in front of the plate, and the batter has slightly less time to react. This probably isn't a meaningful reason for the inside/outside difference, but with a fastball, the extra split-second could help the hitter.
The chart below is shows the run value for fastballs that are put in play by right-handed hitters.
Apparently righties like low and inside fastballs more than lefties, and righties also don't hit fastballs on the outside as well as they hit inside ones.
Looking at all pitch types, right-handed hitters actually hit all down and in pitches very well.
I also wanted to quickly go over the way I calculate the run value for each pitch. I take every event that resulted from a pitch being thrown and assigned it a weight, based on the count it occurred in. Different events are worth more in different counts, and for an extreme example, a 3&0 strike isn't worth as much to the pitcher as a strike thrown in an 0&2 count. By the same logic, any base hit in an 0&2 count hurts the pitcher more than the same hit would have in a 3&0 count. The process and weights are explained a little more in depth here.
There are some loose ends that I need to tidy up, such as if called strikes and swinging strikes should be weighed the same (currently I weigh all strikes, including fouls with less than two strikes, the same amount), and what to do with pitches that result in a steal or caught stealing (currently I'm ignoring this, but a pitcher is partially responsible for the running game, so his pitches should get some penalty/benefit if the runner steals or is caught stealing.)