a high recall is important for a screening
test, which is to be followed up by a more precise test, so that patients with
the disease are not missed out. In contrast a researcher who wants to find a
few actual patients of the disease for further follow up, but is not interested in
finding all patients, may value high precision over recall. Different classifiers
may be appropriate for each of these applications. This issue is explored further
in Exercise 20.5.