Number of Studies	Study Design	Quality of Studies	Consistency	Directness	Other Modifying Factors
N	RCT=High Observational=Low Any other evidence =Very low	Serious limitation (−1) Very serious limitation (−2)	Important inconsistency (−1)	Same uncertainty (−1) Major uncertainty (−2)	Association Strong (+1) Very strong (+2) Dose response gradient (+1) All plausible confounders would have reduced the effect (+1) Imprecise or sparse data (−1) High of reporting bias (−1)

Source: Atkins D, Best D, Briss PA, Eccles M, Falck-Ytter Y, Flottorp S et al. Grading quality of evidence and strength of recommendations. BMJ 2004;328(7454):1490