Diagnosis
How to select and evaluate diagnostic tests?
Study types: Cross-sectional study or a prospective blind comparison to a gold standard.
 

Therapy
Is a treatment effective?
Study Types: Systematic review, randomized controlled trial (RCT), or cohort study

Prognosis
What is the patient's clinical course over time?
Study types: Cohort studies, case studies, or case series

Harm/Etiology
What caused this disease/condition?
Study types: Cohort studies, case studies, or case series can answer question of harm.