I read a fascinating free article about why most published research findings might be false. I am able to follow along almost completely, except for one tiny part.
In it, the author, Ioannidis, writes
Let R be the ratio of the number of “true relationships” to “no relationships” among those tested in the ﬁeld. ... The pre-study probability of a relationship being true is R⁄(R + 1).
He writes in the same paragraph:
Let us also consider, for computational simplicity, circumscribed ﬁelds where either there is only one true relationship (among many that can be hypothesized) or the power is similar to ﬁnd any of the several existing true relationships.
Perhaps this can help explain it, but I do not see how.