Sunday, 17 July 2011

Using Bayes to prove Obama did not write his own book?

There have been many questions about the closeness of President Obama's relationship with Weather Underground terrorist Bill Ayers. A whole new angle on the relationship has been raised in Jack Cashill's book Deconstructing Obama. Using information in this book Andre Lofthus has applied Bayes Theorem to conclude that Bill Ayers actually was the ghost writer for Obama's best selling book Dreams from My Father.

Loftus's analysis is based on a) a comparison of  Dreams with one of Ayers's own books Fugitive Days; and b) a comparison of Dreams with a different book Sucker Punch based on similar material to that of both Dreams and Fugitive Days.

Specifically in a) there were 759 similarities, of which 180 were categorized by Cashill  as "striking similarities", whereas in b) Cashill claims there were just six definite similarities, with a maximum of sixteen possible or definite similarities. As Lofhus's Bayesian analysis is not complete I have done my own analysis here. My own conclusions are not as definite. The evidence does indeed provide very strong support in favour of the books being written by the same author. However, if you have a strong prior belief that the books were written by different authors (say you are 99.9% sure) then even after observing the evidence of 180 striking similarities, it turns out that (with what I believe are more reasonable assumptions than made by Lofhus) there is still a better than 50% chance that the books were written by different authors.

4 comments:

  1. 99.9% prior belief that they were written by different authors? Can you seriously claim that that is not insanely biased in favor or Obama's defense? Arguably there is plenty of prior information suggestive of the scenario that Obama did NOT write his book and Ayers did. But since it is indeed arguable, there is some justification in concluding that there is NO good prior information and hence it is very reasonable to simply use a prior=50%. But let's be really generous (in Obama's favor), put on some rose-colored glasses, and say that his character is one of honesty and so belief (as silly as that is) that there is more chance he wrote the book than not. Even then, going beyond about 90% prior belief that he wrote the book stretches all credulity. Given his history, ideology, and suspiciously unbelievable rise through political ranks, there is at least a very good chance that Ayers wrote the book for him.
    Yet the way you wrote your article, you implied that 99.9% is in some way "reasonable". Come on !

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