LEVEL 1 Evidence from a systematic review or meta-analysis of all relevant RCTs (randomized controlled trial) or evidence-based clinical practice guidelines based on systematic reviews of RCTs or three or more RCTs of goo d quality that have similar results
LEVEL 2 Evidence obtained from at least one well designed RCt
LEVEL 3 Evidence obtained from well-designed controlled trials without randomization
LEVEL 4 Evidence from well designed case-control or cohort studies
LEVEL 5 Evidence from systematic reviews of descriptive and qualitative studies (meta synthesis)
LEVEL 6 Evidence from a single descriptive or qualitative study
LEVEL 7 Evidence from the opinion of authorities and/or reports of expert committees
Different types of clinical questions are best answered by different types of research studies. You might not always find the highest level of evidence (i.e., systematic review or meta-analysis) to answer your question. When this happens, work your way down to the next highest level of evidence.