Another one of the many perturbing errors in Jeffrey's Subjective Probability, this from §4.1:

If I am sure that it was one of the years from 1951 to 1959, with equal probabilities, then my ex(X) will be 1951
9 + 1952
9 + … + 1959
9, which works out to be 1954.5.

One doesn't even have to do any arithmetic (I didn't) to spot the error; for a sequence of an odd number of equally-spaced integers, the average is the middle-most value, in this case 1955.

Tags: bad math, everyday frustrations

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