Table 3.

Levels of evidence for the AOTA Evidence-Based Practice Project

Level	Definition
Design
I	Randomized trial: Comparison of two or more groups or conditions in an experiment with random assignment to group or to sequence of conditions in a repeated-measures design
II	Non-RCT: Comparison of two or more groups or treatments in a quasi-experiment without randomization to group, condition, or sequence
III	Non-RCT: Comparison of one group pre- and post-treatment
IV	Single-subject design: One subject measured at intervals throughout an intervention
N	A Narratives and case studies
Sample size
A	n ≥ 50 persons per condition or group or observations in a single subject design
B	n ≥ 20 persons per condition or group or observations in a single subject design
C	n < 20 persons per condition or group or observations in a single subject design
Internal validity
1	High internal validity: No strong alternate explanation for outcome or other threats to validity, such as attrition, unblinded evaluation, unequal treatment, or spontaneous recovery
2	Moderate internal validity: No strong alternate explanation for outcome but one or two threats to validity exist
3	Low internal validity: Does not meet criteria for 1 or 2
External validity
a	High external validity: Participants represent population and treatment represents current practice or has strong theoretical support and the research was done in a natural (home or clinic) setting
b	b Moderate external validity: Has two of the criteria listed for a
c	Low external validity: Has one or fewer criteria listed for a

AOTA-American Occupational Therapy association; RCT-Randomised controlled trial. Figure taken from Tromblay and Ma [38]