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Thursday, August 18, 2011
Mailbag: Replacement level example calculation
In response to a query:
Whether it’s FIP or ERA or any rate stat, the way to “scale” them the same when you have differing opportunities (IP or plate appearances, etc) is the same:
value = (metric minus baseline) x opportunity
In your case, with FIP (or ERA), we’d like to establish the baseline as something like 1 or 1.25 runs above the league average. So, if the league average is an ERA of 4.00, then our baseline is 5.00 or 5.25. In other words, we create the baseline as the “minimum acceptable level” of performance. Think of a bullpen guy getting a spot start, or a last minute AAA callup, etc.
For opportunity, you use IP, but let’s use IP/9 so that it’s in the form of “games”. This is especially useful since FIP and ERA are also in the form of games (runs per 9 IP).
So, as an example, let’s say you have one guy with a FIP of 3.50 and 180 innings, and you have someone else with a FIP of 2.50 and only 108 innings. What are their values?
We’ll presume a baseline of 5.00.
value = (5.00 - 3.50) x 180/9 = 30
It may not be readily obvious, but the unit here is runs, so we have a pitcher who provided 30 runs of value above the minimum acceptable level (what we normally call the replacement level, or readily available talent level).
For the other guy:
value = (5.00 - 2.50) x 108/9 = 30
As you can see, these two pitchers are equivalent.
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