Table 1 Gray scale of the strength of evidence

I

Strong evidence from at least one systematic review of multiple well-designed, randomized controlled trials.

II

Strong evidence from at least one properly designed, randomized controlled trial of appropriate size.

III

Evidence from well-designed trials without randomization: single, group, pre-post, cohort, time series, or matched case-control studies.

IV

Evidence from well-designed, non-experimental studies from more than one centre or research group.

V

Opinions of respected authorities, based on clinical evidence, descriptive studies or reports of expert committees.