A lot has been written about Stephen Strasburg's impressive MLB debut last Tuesday, but for what it is worth, here's my two cents:As some of you know, the player ability estimates and subsequently all of the predictions created here at baseballcalculus.com, are based upon historical Major League data. Thus, if a player has no Major League experience, we assume he is just an average pitcher. It can get tricky to summarize the prior and posterior distributions that represent this understanding of a player’s ability (see the research link for more on that), but to simplify, the expected OPSA (OPS against – or expected OPS by opposing batters) of any new pitcher is .775 according to these models. Thus, that was Stephen Strasburg’s expected ability according to our models as of Tuesday afternoon. After Tuesday’s game though, most notably the 14 strikeouts with no walks, Strasburg’s expected OPSA immediately fell to .699, moving him from the 446th ranked pitcher in (or out of – since we maintain estimates for retired pitchers) baseball, to the 64th, just ahead of Tim Hudson. A large component of this was his expected K rate jumping from 16.8% to 26.3%. That’s pretty impressive for about two hours work. By comparison, the next best single day change in expected OPSA for a pitcher this season was .039 points by Dodgers rookie John Ely (moving his expected OPSA from .779 to .740) on May 6th after a 7 inning, 4 hit, 1 run, 0 walk, 7 strikeout performance.We’ll be sure to keep track of Strasburg’s projections as he logs more Major league starts. If he can reduced his expected OPSA another .076 points he'll be the top rated starter in the league, and number 3 pitcher overall after Jonathan Broxton and Carlos Marmol.
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