Prospectus Hit and Run... (03/09)
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March 9, 2010 Expanded HorizonsBraves Optimism
The Balance-Sheet Approach One common, back-of-the-envelope way to predict a baseball team’s success is to start with the previous season’s actual record. Then, adding and subtracting for players acquired or lost, one can swap into last year’s results this year’s new expected performances. I call it the balance-sheet approach. Nobody would say this method is deadly accurate, but it is employed quite often during the offseason and spring training by radio hosts and commentators. How good is it? Put simply, it’s terrible. It turns on the implicit assumption that everything except the players gained and lost will perform at exactly the same level they did the year before. Because players (both good and bad) tend to regress toward the mean, such a method will always systematically favor teams that were good last year and punish teams that were bad. Let’s have an example. Last year, the Braves’ actual record was 86-76. This offseason, they traded away Javier Vazquez, acquiring Melky Cabrera (and elite prospect Arodys Vizcaino). They also lost Mike Gonzalez (to free agency) and Rafael Soriano (via trade after he unexpectedly accepted arbitration), but brought on free agents Billy Wagner and Takashi Saito as replacements. They sloughed Kelly Johnson and decided to replace his PAs with Martin Prado’s production. Tallying up the gains and losses in PECOTA-predicted wins (rounded to nearest half-win):
* I have divided Prado’s and Johnson’s numbers in half to reflect the half season that each played as the starter last year. Other part-time players, like Ryan Church, Jesse Chavez, and Jeff Francoeur, have been omitted for simplicity’s sake.
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Team Health Reports: Detroit Tigers
I don't understand equating Prado and Johnson based on the time they were starters. Prado's OPS in the second half of last year, when he was the full-time starter, was .772. Johnson's for the first half, when he was the starter, was .645. Those numbers are pretty far apart.