Bowel anastomoses: The theory, the practice and the evidence base

Table 3 Levels of evidence

Level 1a	Evidence from meta-analysis of randomised controlled trials
Level 1b	Evidence from at least one randomised controlled trial
Level 2a	Evidence from at least one controlled study without randomisation
Level 2b	Evidence from at least one other type of quasi-experimental study
Level 3	Evidence from non-experimental descriptive studies, such as comparative studies and case-controlled studies
Level 4	Evidence from expert committee reports or opinions or clinical experience of respected authorities, or both