Table 1

Common types of clinical question

Type of questionForm of the questionMost reliable study architecture
DiagnosisHow likely is a patient who has a particular symptom, sign or diagnostic test result to have a specific disorder?A cross sectional study of patients suspected of having the disorder comparing the proportion of the patients who really have the disorder who have a positive test with the proportion of patients who do not have the disorder who have a positive test result.
TreatmentIs the treatment of interest more effective in producing a desired outcome than an alternative treatment (including no treatment)?Randomised evidence in which the patients are randomly allocated to receive either the treatment of interest of the alternative (table 2)
PrognosisWhat is the probability of a specific outcome in this patient? A study in which an inception cohort (patients at a common stage in the development of the illness—especially first onset) are followed up for an adequate length of time
AetiologyWhat has caused the disorder? (or, how likely is a particular intervention to cause a specific adverse effect?)A study comparing the frequency of an exposure in a group of people with the disease (cases) of interest with a group of people without the disease (controls)—this may be a randomised controlled trial, a case control study, or a cohort study.