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THE BOOK--Playing The Percentages In Baseball

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Monday, March 05, 2012

Cherry picking quiz…

By .(JavaScript must be enabled to view this email address), 05:37 AM

From the Sloan thread, where we were discussing handicapping touts and cherry picking results…

Here is another MGL quiz:

Let’s say that you make 200 bets a year for 10 years and your “true” winning percentage is 50%.  IOW, you flip 200 coins per year for 10 years. You publish your yearly results. Someone doesn’t like you and they want to make sure that everyone know that you are a loser. They decide to be (somewhat) truthful and publish some or all of your results. They decide to choose your results anywhere from “the last 3 years” to “all 10 years”. He doesn’t want to choose the last year or 2 years because everyone will say that that could be just a fluke.

What are the chances that he can find a subset of your results, limited to “last 3 years,” “last 4 years,” all the way to “last 10 years,” such that you are indeed a loser, despite being 50/50 (theoretically) in actuality?

What about if you have a true 52% probability of winning?

55%?

Same question, but the guy decides that he’ll use 1 or 2 years as well if he has to?

(8) Comments • 2012/03/06 SabermetricsSamplingStatistical_Theory