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31?

With the Fake Teams Head-to-Head league drafting in less than two weeks, I think this brief article about the format would be interesting.  It ran in USAToday a couple weeks ago, and here is the link H2H.  I suggest anyone who is particpating in a head-to-head league for the first time click through and read it.

As a bonus, there is a list of pitchers who could be expected to do worse/better based on an expected ERA calculation.

A pitcher's slugging and on-base percentages allowed gives a good picture of performance beyond just ERA. Olkin found that multiplying slugging and on-base and then multiplying the result by 31 gives a number that usually is close to a pitcher's ERA.

Does anyone know why 31 is used?  Is it just because a measure of OBP multiplied by a measure of SLG gives a result that is close to the league average ERA when multiplied by 31?  Could it be that spurious or is there something more meaningful?