Friday, 26 April 2013

Errors in the book (including interesting Binomial distribution error)

Eagle-eyed readers continue to point out errors in the book and we regularly update the Errata page here with the changes:

An especially interesting error (with a fair few ramifications) has been discovered in Example 5.6 on pages 122-23.  The error was triggered in our calculation of the 99th percentile of the particular Binomial distribution there. Basically we wanted to know the following:
what is the number k for which the probability of tossing at least k heads in 100 tosses is 0.01?
Now, because the Binomial is a discrete distribution there is no reason why there should be an integer k for which the probability of tossing at least k heads in 100 tosses is exactly 0.01. In fact it turns out that:
  • for k=61 the probability is 0.017
  • for k=62 the probability is 0.0105
  • for k=63 the probability is 0.006
So, in the original manuscript we just used k=62 because that was easily the closest to 0.01.

However, to make the point in the example to work it was important that the probability was LESS than 0.01. So, whereas the example used k=62, what we strictly needed was k=63. This meant all the subsequent calculations were wrong.
There were also some other errors in the example including a couple that had been introduced by the publishers as they were correct in our final manuscript.

1 comment:

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