TABLE II.

Levels of evidence and recommendation grades according to the Canadian Task Force on Preventive Health Care^a

Levels of evidence
i	Evidence from one or more randomized controlled trials
ii-1	Evidence from controlled trials without randomization
ii-2	Evidence from cohort or case–control analytic studies, preferably from more than one centre or research group
ii-3	Evidence from comparisons between times or places with or without the intervention (dramatic results in uncontrolled experiments could be included here)
iii	Opinions of respected authorities, based on clinical experience; descriptive studies or reports of expert committees
Recommendation grades
a	There is good evidence to recommend the action.
b	There is fair evidence to recommend the action.
c	The existing evidence is conflicting and does not allow for a recommendation for or against use of the action; however, other factors might influence decision-making.
d	There is fair evidence to recommend against the action.
e	There is good evidence to recommend against action.
i	There is insufficient evidence (in quantity or quality, or both) to make a recommendation; however, other factors might influence decision-making.

^aAvailable at http://www.canadiantaskforce.ca.