One out of 500 people has disease D. You're a doctor and suspect that one of your patients may have that disease. You order a diagnostic test T for that patient. The test is not perfect - it accurately claims the disease is "present" in 80% of the patients who actually have it, and accurately declares the disease as "absent" in 70% of the patients who indeed don't have the disease.
a. If the test result comes back negative (the test says "absence of disease"), what does that mean for your patient? How likely is he to actually have disease D?
b. How about if the test comes back positive (the test says "disease")?