Table 3.
The 2011 Oxford centre for evidence-based medicine levels of evidence [28]
| Question | Level 1a | Level 2a | Level 3a | Level 4a | Level 5 |
|---|---|---|---|---|---|
| How common is the problem? | Local and current random sample surveys (or censuses) | Systematic review of surveys that allow matching to local circumstancesb | Local non-random sampleb | Case-seriesb | 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 standardsb | Case–control studies, or “poor or non-independent reference standard”b | 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 randomized triala | Case-series or case–control studies, or poor quality prognostic cohort studyb | n/a |
| Does this intervention help? (Treatment Benefits) | Systematic review of randomized trials or n-of-1 trials | Randomized trial or observational study with dramatic effect | Non-randomized controlled cohort/follow-up studyb | Case-series, case–control studies, or historically controlled studiesb | Mechanism-based reasoning |
| What are the COMMON harms? (Treatment Harms) | Systematic review of randomized 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 randomized trial or (exceptionally) observational study with dramatic effect | Non-randomized 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.)b | Case-series, case–control, or historically controlled studiesb | Mechanism-based reasoning |
| What are the RARE harms? (Treatment Harms) | Systematic review of randomized trials or n-of-1 trial | Randomized trial or (exceptionally) observational study with dramatic effect | |||
| Is this (early detection) test worthwhile? (Screening) | Systematic review of randomized trials | Randomized trial | Non-randomized controlled cohort/follow-up studyb | Case-series, case–control, or historically controlled studiesb | Mechanism-based reasoning |
a
Level may be graded down on the basis of study quality, imprecision, indirectness (study PICO does not match questions PICO), because of inconsistency between studies, or because the absolute effect size is very small; level may be graded up if there is a large or very large effect size
b
As always, a systematic review is generally better than an individual study