P(disease | test) is not the same for everyone; it is context dependent. If you know that you are at high risk for a disease to begin with, Bayes’s rule allows you to factor that information in. Or if you know that you are immune, you need not even bother with the test! In contrast, P(test | disease) does not depend on whether you are at high risk or not. It is “robust” to such variations, which explains to some degree why physicians organize their knowledge and communicate with forward probabilities. The former are properties of the disease itself, its stage of progression, or the sensitivity
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