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Tuesday, April 28, 2009
Money in the bank counts, when looking at your destination
We spoke last week about Dave’s article regarding the Mariners’ hot streak, how those wins are already in the bank, so you need to forecast the future. I think Rob Neyer must have missed it. Seeing that Rob is on top of these things and is a smart cookie, I have to believe that there are thousands or millions of people out there who are making the same mistake.
Rob is talking about two things:
1. Ellsbury stole 50 bases last year, on 609 PA. He’s using this as his prior, regardless of what he has done this year. Not a great choice, but let’s live with it.
2. The Redsox SB record is 54. Ellsbury already has 10. That means he needs 44 more to tie. Presuming a 50/609 rate, that means he needs 536 more PA to tie. Add in the 81 PA he had at the time of the writing, and that means he’ll get to 54 SB after 617 PA.
Rob though said almost 660 PA, since he did 54/50*609. You see, what Rob said only makes sense if it was made prior to Opening Pitch, or if the number of SB Ellsbury had in the bank was 6 or 7 SB (50/609*81). Once we know that Ellsbury has more in the bank than expected (10), he needs fewer SB in the future to tie it up.
Or, if you want to make it clearer, suppose that Jacoby has 53 SB after 54 games. Let’s presume his prior remains the same (50 SB per 609 PA). By using Rob’s methodology, he would still need 660 PA to tie the record. That’s because he didn’t include the 53 he already had in the bank, and needed only one more SB to tie the record. Indeed regardless if Jacoby had 0 or 53 SB, and whether he had 0 or 659 PA, he’ll always need 660 based on the flawed reasoning.