Table 1.

The modified Oxford's level of evidence scale.

Level	Type of study
I	Meta-analysis, systematic reviews of RCTs, randomized controlled clinical trials (RCTs)
II	Systematic review of cohort studies, systematic reviews of clinical trials, individual cohort studies, individual clinical trials, quasiexperimental
III	Systematic review of case-control studies, individual case-control studies
IV	Systematic reviews of mixed low evidence level studies. Case series, case reports, and cross-sectional studies
V	Expert opinions without explicit critical appraisal, experimental research, animal studies, reviews