Youden's J is the likelihood of a positive test result in subjects with the condition
versus those without the condition. It is also the probability of an informed decision (as opposed
to a random guess).
Youden's J index combines sensitivity and specificity into a single measure (Sensitivity +
Specificity - 1) and has a value between 0 and 1. In a perfect test, Youden's index equals 1. It
is also equivalent to the vertical distance above the diagonal no discrimination (chance) line to
the ROC curve for a single decision threshold.