[tw1010]

Not really, it's one of the most common mistakes I see in bayesian calculations. If the author was basing a lengthy series of calculations on that first step, it would be worse (but in this case the expression is quickly replaced by a corrected version for the classification discussion).

[toxik]

Do you have data to support that it’s the most common mistake? It seems obvious to me from P(A|B) = P(A, B)/P(B) and P(A, B) = P(B|A) P(A)

[tw1010]

I didn't say it was one of the most common mistakes, I said it's one of the most common mistakes I have observed. Purely subjective.

[macleginn]

I have a similar experience, but precisely because of this I am very careful to check the formula each time I have to type it. Mistakes in the formulas are not "just typos", they are a very annoying and potentially harmful kind of typos, and we must take great care in order to avoid them.

[tw1010]

Like I said above, "typos" in sequences of calculations are obviously problematic, and lead (almost always) to mistakes in the final result. In this case that's not applicable since there's no "second arithmetic manipulation" following the typo:ed one. (The author replaces the incorrect one with the correct bayesian equation in the next section.)