Table 1.

Level-of-evidence definitions.

Level I	Systematic reviews, meta-analyses, randomized controlled trials
Level II	Two groups, nonrandomized studies (e.g., cohort, case–control)
Level III	One group, nonrandomized (e.g., before and after, pretest and posttest)
Level IV	Descriptive studies that include analysis of outcomes (e.g., single-subject design)
Level V	Case reports and expert opinions, including consensus and narrative literature review)

From Sackett, Rosenberg, Muir Gray, Haynes, and Richardson (1996), p. 71.