The discussion a month ago about Matt Cain and his –and teammates’—home run suppression had me thinking about park factors and the nature of comparing unadjusted stats like RA to ones like xFIP. It feels at times that people will point to the gap between a pitcher’s ERA and his FIP or xFIP as proof that said pitcher is a statistically significant outlier at having control over one of the results of pitching that the DIPS theory rejects. Where I think a disconnect forms is that those assertions ignore that controlling for “luck” factors like BABIP and HR/FB is not the only thing that metrics like FIP do. They also attempt to control out the pitcher’s home park and the defensive skill behind him.
If we want to examine the question of whether a metric like xFIP underrates Matt Cain (or anyone) because it ignores some aspect of Cain’s skill at controlling his batted balls then we need to first isolate that part of the spread between his ERA and his xFIP. That means making corrections for park and defensive skill.
I started off only intending to look at park factors for home runs in this case because a large part of the argument is that Cain (or the Giants’ staff) does better than expected at preventing keeping fly balls in the park. And I wanted to go into greater depth than simply using San Francisco’s park factor on the entirety of Cain’s outcomes so I separated every fly ball Matt Cain gave up by the park it occurred in and whether the batter was hitting from the left or right side at the time. I feel the latter is important because according to my research on handed park factors, parks (San Francisco’s is one) can have vastly different effects on right and left-handed batters. On the whole, Cain’s average fly ball has occurred in a park that decreases HR/FB rate by about 6%. Over his career, that’s about six additional home runs or about 8.5 runs.
That’s not a lot to be true, but that’s only one park factor and only one thing out of a pitcher’s control. Another is the quality of his defense and that is an area that Matt Cain has benefitted from tremendously. From 2006 (Cain’s first full season) through last season, the Giants have totaled a league-leading 193 UZR. The next highest mark is the Cubs at 116 runs. The Giants’ run of defensive excellence the past five seasons might be one of the more under talked about streaks in baseball. Assuming that Cain gets a share of the defensive bonus equal to the proportion of innings he’s pitched, Cain is in debt to his fielders at a total of just under 30 runs.
Adjusting Cain’s runs allowed for things we can all agree on, parks affect home run rates and defenses affect BABIP, raises his RA by about 0.3 runs. Other factors that xFIP glosses over that ERA doesn’t, like opposing hitter quality, shows that the average hitter Matt Cain has faced has a slugging percentage about 10 points lower than the National League average over the same time. A difference that looks to be worth about another 0.1 runs allowed average. It helps to get to face the Padres and Dodgers often.
All together, adjusting for park home run rates, defensive skill and quality of batters faced cuts the gap between Matt Cain’s ERA and his xFIP by about half. True, that still leaves a rather sizable difference of about a half run per nine innings. Identifying the cause of the remaining gap is not my intent here however. Rather, I only wanted to point out that when it comes down to such a debate, it should not be ERA versus xFIP. You can try to argue that xFIP is not valuing Matt Cain fairly because he possess some extra skill that xFIP is not capturing. But you cannot use the gap between ERA and xFIP to prove it.
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