@MeBigbrain @matty @sickburnbro
You split the two groups into "test positive" and "test negative" then, based on rates of false positive/negative, you split those groups into a further two groups

So you have: % positive = % actual positive + % false positive

And: % negative = % actual negative + % false negative

You use the decision tree to calculate total probability of X occurrence, and you use that number as a guide for decision making.