Table 2 Oxford Centre for Evidence-based Medicine (OCEBM) levels of evidence (2011) [45]
Question | Step 1 (level 1*) | Step 2 (level 2*) | Step 3 (level 3*) | Step 4 (level 4*) | Step 5 (level 5) |
|---|---|---|---|---|---|
How common is the problem? | Local and current random sample surveys (or censuses) | Systematic review of surveys that allow matching to local circumstances** | Local non-random sample** | Case-series** | N/A |
Is this diagnostic or monitoring test accurate? (diagnosis) | Systematic review of cross-sectional studies with consistently applied reference standard and blinding | Individual cross-sectional studies with consistently applied reference standard and blinding | Non-consecutive studies or studies without consistently applied reference standards** | Case-control studies or poor or non-independent reference standard** | Mechanism-based reasoning |
What will happen if we do not add a therapy? (prognosis) | Systematic review of inception cohort studies | Inception cohort studies | Cohort study or control arm of randomised trial* | Case-series or case-control studies or poor-quality prognostic cohort study** | N/A |
Does this intervention help? (treatment benefits) | Systematic review of randomised trials or n-of-1 trials | Randomised trial or observational study with dramatic effect | Non-randomised controlled cohort/follow-up study** | Case-series, case-control studies or historically controlled studies** | Mechanism-based reasoning |
What are the common harms? (treatment harms) | Systematic review of randomised trials, systematic review of nested case-control studies, n-of-1 trial with the patient you are raising the question about, or observational study with dramatic effect | Individual randomised trial or (exceptionally) observational study with dramatic effect | Non-randomised controlled cohort/follow-up study (post-marketing surveillance) provided there are sufficient numbers to rule out a common harm. (For long-term harms the duration of follow-up must be sufficient.)** | Case-series, case-control or historically controlled studies** | Mechanism-based reasoning |
What are the rare harms? (treatment harms) | Systematic review of randomised trials or n-of-1 trial | Randomised trial or (exceptionally) observational study with dramatic effect | |||
Is this (early detection) test worthwhile? (screening) | Systematic review of randomised trials | Randomised trial | Non-randomised controlled cohort/follow-up study** | Case-series, case-control or historically controlled studies** | Mechanism-based reasoning |