Level of evidence

Level of evidence determines the current clinical practice:

1a: systemic reviews of randomised controlled trials

1b: individual randomised controlled trial with narrow confidence interval

1c: all or none randomised controlled trial

2a: cohort studies

2b: individual cohort study or low quality randomised controlled trial

2c: "outcomes" research, ecological studies

3a: case-control studies

3b: individual case-control study

4: case series

5: expert opinion

In the past, we rely on expert opinion on the courts but it is biased. Nowadays, the gold standard of evidence is randomised controlled trial.