Diagnosis questions are usually about the degree to which a particular test is reliable or clinically useful, generally asked in order to decide whether a patient would get enough benefit from the test, on average, to justify having it done. Most articles on diagnosis compare the results of the diagnostic test being studied to the results of another standard test that is regarded as being a definitive "gold standard" test.
EXAMPLE
You may be comparing, for example, tests that are used to diagnose Alzheimer's Disease.
In diagnosis questions, these kinds of questions arise: