TABLE 1.

Level of evidence used to rate the design of each study (adapted from the Oxford Centre for Evidence-Based Medicine March) (38).

Level of evidence	Study design
1	Well-designed and conducted RCT
2	Cohort studies and low quality RCT (e.g., <80% follow-up)
3	Case-control studies
4	Case-series and poor quality† cohort studies or case-control studies

RCT, randomized controlled trials.

^†Poor quality cohort study indicate one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both exposed and non-exposed individuals and/or failed to identify or appropriately control known confounders and/or failed to carry out a sufficiently long and complete follow-up of patients; poor quality case-control study indicate one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both cases and controls and/or failed to identify or appropriately control known confounders.