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Design
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I
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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
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II
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Non-RCT: Comparison of two or more groups or treatments in a quasi-experiment without randomization to group, condition, or sequence
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III
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Non-RCT: Comparison of one group pre- and post-treatment
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IV
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Single-subject design: One subject measured at intervals throughout an intervention
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N
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A Narratives and case studies
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Sample size
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A
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n ≥ 50 persons per condition or group or observations in a single subject design
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B
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n ≥ 20 persons per condition or group or observations in a single subject design
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C
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n < 20 persons per condition or group or observations in a single subject design
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Internal validity
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1
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High internal validity: No strong alternate explanation for outcome or other threats to validity, such as attrition, unblinded evaluation, unequal treatment, or spontaneous recovery
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2
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Moderate internal validity: No strong alternate explanation for outcome but one or two threats to validity exist
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3
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Low internal validity: Does not meet criteria for 1 or 2
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External validity
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a
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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
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b
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b Moderate external validity: Has two of the criteria listed for a
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c
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Low external validity: Has one or fewer criteria listed for a
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