Table 1.

Level of scientific evidence and gradation of studies.

Level of scientific proof provided by the study	Quality grade
Level 1 •Meta-analyses of randomized controlled trials •Randomized controlled trials of high power	A Established scientific proof
Level 2 •Randomized controlled trials of low power •Properly conducted non-randomized controlled trials •Cohort studies	B Scientific presumption
Level 3Case-control studies	C Low-level of scientific proof
Level 4 •Comparative studies with major bias •Case series