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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 |
