Environmental risk factors for inflammatory bowel diseases: Evidence based literature review

Table 2 Oxford Centre for Evidence-Based Medicine evidence levels of evidence scale

Level	Study questions on therapy/prevention, etiology/harm
1a	Systematic review (with homogeneity of RCTs)
1b	Individual RCT (with narrow confidence intervals)
1c	All or none studies
2a	Systematic review (with homogeneity) of cohort studies
2b	Individual cohort study (including low quality RCT, e.g., < 80% follow-up)
2c	Outcomes research: ecological studies
3a	Systematic review (with homogeneity) of case control studies)
3b	Individual case control study
4	Case-series (and poor quality cohort an case control studies ++)
5	Expert opinion without explicit critical appraisal or based on physiology, bench research or first principles

Users can add a minus-sign “-” to denote the level of evidence that fails to provide a conclusive answer because: either a single result with a wide confidence interval, or a systematic review with troublesome heterogeneity. Provided by Ref. [7].