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Monday, January 03, 2011
MLE of groundball rates
Good stuff from Clay as he shows us that the out rates via GB decreases from 51.4% in AAA to 48.2% in MLB for pitchers in the same year.
He does make a mention that he’s talking about GB outs and not all GB (including hits). He correctly notes that there is scorer bias in tracking whether a hit is a ground or air hit.
Now, is it possible for the pitcher to give up the same number of groundballs, but have a decrease in groundball outs? Well, yes, but it is probably unlikely. Suppose that the fielding talent of players increases in MLB disproportionately toward outfielders. Or, parks are configured so that it’s easier to get a FB out in MLB. Or, hitters faced by pitchers in MLB are more more likely to get an out on air balls than in the minors. What would happen? Here’s one scenario, and let’s say these are all known to be true:
100 balls in minor league parks
14 GB hits
36 GB outs
16 Air hits
34 Air outs
100 balls in MLB parks (same pitchers, different hitters, different parks)
16 GB hits
34 GB outs
14 Air hits
36 Air outs
In this illustration, you have 50 GB and 50 Air balls in minors and majors. The difference is that it’s easier to get a GB hit in MLB, and easier to get an air out in MLB. The result is that by only looking at ground out to all outs, you get 51.4% GB outs in the minors and 48.6% GB outs in MLB. Even though the actual distribution of GB and Air balls was identical.
I’m not saying this is what happens. I’m just saying that by throwing out some data due to concern of scorer bias, you are introducing another potential bias. I doubt the impact would be anywhere near as what my illustration would purport. I think it would be good to also seeing the all the data as well, so we can see both side by side (rates of GB and rates of GB outs).
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