Table 2.

Levels of evidence key.

Level	Studies in Each Category
I	Large randomized trials with clear-cut results; low risk of false-positive error (_) and false-negative (_) error
II	Small, randomized trials with uncertain results; moderate to high risk of false-positive (_) and/or false negative (_) error
III	Nonrandomized, contemporaneous controls
IV	Nonrandomized, historical controls
V	Case series, uncontrolled studies, expert opinion

Adapted from Sackett DL⁶²