Table 1 Grades of Evidence

A

Systematic reviews of randomized controlled trials and/or randomized controlled trials.

B

Systematic reviews of observational studies and/or high-quality observational studies including cohort and case-control studies.

C

Case-series, case-reports and/or poor quality cohort and case-control studies.

D

Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles."