Table 1.

Sackett's Levels of Evidence²⁵

1A	Systemic Review of Randomized Controlled Trials
1B	RCTs with Narrow Confidence Interval
1C	All or None Case Series
2A	Systemic Review Cohort Studies
2B	Cohort Study/Low Quality RCTs
2C	Outcomes Research
3A	Systematic Review of Case-Controlled Studies
3B	Case-controlled Study
4	Case Series, Poor Cohort Case Controlled
5	Expert Opinion

Abbreviations: RCTs, randomized controlled trials