[0] Strong [1] Studies of strong design Free from design flaws, bias, and execution problems [2] Findings generally consistent in direction and size of effect or degree of association, and statistical significance with minor very exceptions [3] One large study with a diverse population or several good quality studies Large number of subjects studied Studies with negative results have sufficiently large sample size for adequate statistical power [5] Studied population, intervention and outcomes are free from serious doubts about generalizability [4] 
Studied outcome relates directly to the question Size of effect is clinically meaningful Significant (statistical) difference is large