Category A: Supportive Literature. Randomized controlled
trials report statistically significant (P 0.01) differences between
clinical interventions for a specified clinical outcome.
Level 1: The literature contains multiple randomized controlled
trials, and the aggregated findings are supported by
meta-analysis.‡
Level 2: The literature contains multiple randomized controlled
trials, but there is an insufficient number of studies to
conduct a viable meta-analysis.
Level 3: The literature contains a single randomized controlled
trial.