|Type of Study|
|A||1a||Systematic review of (homogeneous) randomized
|A||1b||Individual randomized controlled trials (with narrow
|B||2a||Systematic review of (homogeneous) cohort studies
of "exposed" and "unexposed" subjects
|B||2b||Individual cohort study / low-quality randomized
|B||3a||Systematic review of (homogeneous) case-control studies|
|B||3b||Individual case-control studies|
|C||4||Case series, low-quality cohort or case-control studies|
|D||5||Expert opinions based on non-systematic reviews of
results or mechanistic studies
Critically-appraised individual articles and synopses include:
Two things to remember:
1. Studies in which randomization occurs represent a higher level of evidence than those in which subject selection is not random.
2. Controlled studies carry a higher level of evidence than those in which control groups are not used.