What determines batters’ selectivity levels?
by Craig GlaserMarch 15, 2010
If you haven't read my previous article "The influence of batters' expectation on pitch perception" you might want to start there as this article is very much a continuation of that one. In that article I looked at what I believe is the influence of a batter's expectations about the likelihood of a type of pitch being thrown on their ability to discriminate balls from strikes. We saw evidence that batters are aware of how frequently pitchers throw each type of pitch on each count and that these beliefs influence their approaches at the plate.
I'm back today to take a look at the other side of the story - selectivity. As a quick refresher, batting eye is a measure of a batter's ability to discriminate between balls from strikes. It is maximized by a batter who swings at a lot of pitches in the strike zone and very few pitches out of the strike zone. Selectivity on the other hand is a measure of bias. It tells us whether batters are more concerned with misses (not swinging at a strike) or with false alarms (swinging at a ball). When selectivity is zero batters will make these errors at an equal rate. When it is positive the batter will swing less frequently overall and have a higher miss rate than false-alarm rate. Conversely when it is negative the batter will swing more frequently with a higher false-alarm rate and a lower miss rate. When you look at individual batters there does not seem to be much of a correlation between these two measures, though when you look at the league grouped by pitch count and pitch type there does seem to be a weak correlation between the two measures.
While this correlation is small it does make it more difficult to figure out exactly which measures are causally related to each other. Batting eye was most correlated with percentage of pitch type thrown, implying that batters take into account what type of pitch is coming and are better at judging the pitch types which are more frequent on that count. This also makes logical sense to me. We did see a break from the pattern in change-ups which is interesting and which we’ll come back to later.
Selectivity on the other hand is most correlated with percentage of strikes thrown - another link which makes logical sense. As pitchers throw more strikes it becomes beneficial for the batter to swing more often. Since there will be more opportunities to miss and less to false alarm, a batter would be smart to try to lower their miss rate at the expense of raising their false-alarm rate. They can achieve this by lowering their selectivity, which would lead to less errors overall. Below is a graph showing z-scores of percentage of pitches thrown for strikes compared to z-scores of selectivity broken down by pitch type and count. It does seem as percentage-of-strikes thrown relates closely to selectivity level.
There are, however, a small number of rather large outliers here. Looking at the data, I found a very similar phenomenon to one which we saw in the batting eye article - they all belong to one type of pitch. Here is the same graph without curveballs included.
While the lack of correlation in curveballs is puzzling, we see that there is an incredibly strong correlation for the other three pitches. I then went back to the batting eye article where change-ups do not demonstrate the expected correlation with percentage of change-ups thrown.
There is an almost perfect negative correlation between batting eye and percentage of change-ups thrown for strikes! This is puzzling and is not an adjustment which the batter would like to make. We know that batting eye on change-ups is not very heavily influenced by percentage of change-ups thrown so what is going on here? Perhaps it has something to do with the fact that change-ups are generally the slowest, straightest and easiest pitches to read? Batters might find it less important to sit on change-ups since they feel that they have more time to adjust to them. However the more they sit on the fastball the more effective the change-up becomes. Pitchers can take advantage of this fact by throwing change-ups later in the count when they are harder to read. Unfortunately I have not found a good correlation with curveball selectivity yet. Perhaps the difficulty that batters have in judging curveballs hinders their abilities to properly adjust their selectivity?
It seems to me that batters use both of these pieces of information (and probably other information as well) to adopt the approach they feel is most advantageous for that type of pitch. This could lead to batters applying different weights to their prior beliefs in order to best prepare for each count. I think this is a really interesting question, one which I don't have an answer for yet.
I am looking forward to doing more research in this area and hearing feedback and theories from other people as well. I believe I have demonstrated that batters apply their knowledge of pitcher tendencies to alter their batting approach and that I have found a good way to measure these changes. The specifics are based on correlation and intuition but I do believe that there is strong evidence to support the conclusion. Looking at individual batters, individual pitchers, pitch sequences and other data should provide a clearer picture and an interesting insight to batters’ beliefs and approaches to hitting.
Read more of Craig's work on his website Sabometrics
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