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How much credit does Mike Mussina deserve for pitching in the AL East?

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Mike Mussina is a Hall of Fame candidate to begin with, but people seem to give him a boost for pitching in the AL East. How big of a boost should that be?

The Star-Ledger-USA TODAY Sports

This, admittedly, is not a piece that will advocate for Mike Mussina to be a Hall of Famer. I absolutely believe that Mussina should be a Hall of Famer, and I will go as far to say that he was one of the best pitchers of his generation. What I'm going to tackle here is certainly related to his Hall of Fame case, but a bit more nuanced.

In a recent piece for The Sporting News, Jesse Spector argues that the one factor that will and should keep Mussina out of the Hall is his record-high ERA, a mark that would be the highest of any Hall of Fame pitcher after World War II. On the other side of the argument is Sports Illustrated writer Jay Jaffe, who shows that by his JAWS model, Mussina is an absolute shoe-in; he also points out (on Twitter) that the ERA argument should be in terms of ERA+, a statistic that is league and park adjusted and puts him about 20% better than the league for his entire career.

But even for those who side with Jaffe (like myself), the argument often drifts towards his division. Because he pitched in an inflated offensive environment in the AL East, even his ERA+ is too conservative. Many would then contend that 20% better than league average is not enough. How much better is it, then?

To figure this out, I took a look at the Baseball-Reference's Play Index pages for Mussina and looked at both AL East opponents, as well as the opponents he faced the most, because he did begin his career before the divisional era. It didn't matter either way, because he induced an opposing OPS of .702 for AL East opponents and a .703 OPS for his most faced opponents. I can't really do anything with that by itself, so it only makes sense if we look at a comparison who faced different opponents but had similar park-adjusted results. That pitcher is Curt Schilling.

Schilling pitched (full time) from 1992 to 2007, and Mussina pitched from 1991 to 2008. Schilling had a career ERA- of 80, and Mussina had one of 82. The main difference is, though, they faced vastly different opponents. Schilling faced teams in the National League in a slightly lower offensive environment; the Giants, Expos, Braves, Rockies, and Dodgers were his primary adversaries. While Mussina's AL East had an average wRC+ in his career of 101.2, Schilling's main rivals had an average wRC+ of 93.5. To no one's surprise, Schilling fared well against these opponents as they only put up a .660 OPS. To what extent did Schilling have an advantage; are their ERA-'s as comparable as they seem?

They still are, for the most part. Mussina's and Schilling's ERA differ by 6.1674%, and their OPS's against their rivals differ by... 6.1625%. Heh. Their run environments, though, differ by about 8.2%, so there seems to be a difference between the results the pitchers are obtaining and the run environments they're in. We would normally expect Mussina to have the opposing OPS's differ by that same percentage as the run environment, but I'm going to say that that's within the margin of error as unadjusted OPS, the best number one can use in the Play Index, is not going to give you the same result as wRC+.

The advantage that Mike Mussina may have gained from playing in the AL East is actually not as great as many people would believe. I initially came into this thinking that Mussina would endure a massive deduction against AL East opponents, but that just wasn't the case. Mussina did do slightly worse against AL East opponents compared to his career (less than 10 OPS points) and Curt Schilling did do slightly better against his rivals compared to his career (13 points of OPS), but it frankly wasn't enough to significantly skew ERA- one way or the other. The effect it caused could have been on the order of about 2%, but I would say that that's much less than the "mental boost" we give Mussina as being attached to his division.

This is probably because when we think of a player "in" a division, we often forget that they don't play most of their games against their division, so the difference between being in a division and out of it is only the difference of a few games a season, and that's even fewer for a starter who doesn't play every game. The offensive environment and league certainly matter, but "sub-offensive environments" of divisions are much less pronounced. Mussina did play a lot against the Tampa Bay Rays, too, you know.

Regardless of this discovery, Mussina in my mind is still a Hall of Fame pitcher. He still has an ERA in a neutralized environment of 3.19, and his fWAR, rWAR, and JAWS put him among the best starting pitchers of all time. But, let's not pretend that he should somehow get more credit for pitching where he did. ERA+ and ERA- do an excellent job of doing that dirty work for us, and it nearly tells the full story: Mussina was an amazing pitcher, and he would have been just as amazing anywhere he pitched.