Talk:Pre- and post-test probability

Motivation for having own article

I forked this section from Likelihood ratios in diagnostic testing, partly to provide a common fork som that one and positive predictive value, and partly because so many incoming links (such as positive pre-test probability, negative post-test probability, negative post-test odds etc) cannot feasibly be redirected to a section. Mikael Häggström (talk) 08:30, 30 January 2011 (UTC)

Confusion about example

I'm confused about why go through all the rigamarole with odds and likelihood ratios, etc in the given example.

What I take it we're after is the post-test probability. I.e., what we want to know is $P(Cancer=True|Test=True)$.

But that can be read directly off of the chart given in the article, in one calculation, by the definition of conditional probability:

$P(Cancer=True|Test=True)={\frac {P(Cancer=True\land Test=True)}{P(Test=True)}}={\frac {2}{2+8}}=0.10$

That seems way easier than the complicated 4 step process described in the example. So why would you ever do it that way? — Preceding unsigned comment added by Quadari (talk • contribs) 14:26, 23 February 2011 (UTC)