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Tuesday, July 12, 2011
wOBA to ERA
Lee does a conversion from wOBA to ERA. However, he does a linear conversion, which is wrong. Since wOBA is Linear Weights, wOBA should not be used so simply, for pitchers. Pitchers should use BaseRuns.
If you insist on using wOBA, then you need to do something more multiplicative. My method is to do the following:
1. wOBA / (1-wOBA)
2. Take result, and raised to power of 1.5
3. Take result, and multiply by a constant (12 for ERA or 13 for RA9)
You can play around with the 1.5 and 12 and 13 to match to that season.
But, the BETTER thing to do is to use BaseRuns. And, theoretically, you should not require any fudge factor on a seasonal level, since giving up 15 HR, 50 walks, and getting 200 K on 220 IP should give you the same answer, whether in 2010 AL, 1985 NL, or 1969 high school. Now, what is missing is all the little things, like base stealing, advancing the extra base, etc. The fudge factor would change to account for the missing parameters. So, in that respect, you could have different results in these various leagues.
Anyway, if you want to get it right, use a simulator. If you want to get it fast but be in great shape, use BaseRuns. Everything else is a trick or shortcut, meaning it will have limitations.
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