Levels of evidence and grades of recommendations
| Level or grade | Description |
|---|---|
| Level of evidence | |
| I | Evidence demonstrated by means of meta-analyses of well-designed controlled trials or large randomized trials with clear-cut results (low rate of false-positive and false-negative errors, high power) |
| II | Evidence demonstrated by means of small randomized trials with uncertain results (high rate of false-positive and false-negative errors, low power) |
| III | Evidence demonstrated by means of nonrandomized concurrent cohort comparisons with contemporaneous controls |
| IV | Evidence demonstrated by means of nonrandomized historical cohort comparisons |
| V | Evidence demonstrated by means of case series without controls |
| Grade of recommendation | |
| A | Supported by Level I evidence or multiple Level II, III or IV trials presenting concordant observations |
| B | Supported by Level II, III or IV trials presenting generally concordant observations |
| C | Supported by Level II, III or IV trials presenting nonconcordant observations |
| D | Supported by few trials or no empiric evidence |
Adapted from Cook and colleagues13 with permission from the American College of Chest Physicians.