Levels and strength of evidence

Level

Type of Evidence

1A

Systematic review of homogeneous RCTs (similar population, intervention, etc.) with or without meta-analysis

1B

Well-designed individual RCT (not a pilot or feasibility study with a small sample size)

2A

Systematic review of cohort studies

2B

Individual prospective cohort study, low-quality RCT (e.g., <80% follow-up or low number of participants; pilot and feasibility studies); ecological studies; and two-group, nonrandomized studies

3A

Systematic review of case-control studies

3B

Individual retrospective case-control study; one-group, nonrandomized pre-posttest study; cohort studies

4

Case series (and low-quality cohort and case-control study)

5

Expert opinion without explicit critical appraisal

Strength of Evidence (Level of Certainty)

Description

Strong

• Two or more Level 1A/B studies
• The available evidence usually includes consistent results from well-designed, well-conducted studies. The findings are strong and they are unlikely to be strongly called into question by the results of future studies. (AOTA review parameters: Two or more Level 1 studies)

Moderate

• At least one Level 1A or Level 1B high-quality study or multiple moderate-quality studies (Level 2A/B, Level 3A/B, etc.)
• The available evidence is sufficient to determine the effects on health outcomes, but confidence in the estimate is constrained by such factors as:
  • The number, size, or quality of individual studies
  • Inconsistency of findings across individual studies.

As more information (other research findings) becomes available, the magnitude or direction of the observed effect could change, and this change may be large enough to alter the conclusion related to the usefulness of the intervention. (AOTA review parameters: At least one Level 1 high-quality study or multiple moderate-quality studies)

Low

• Small number of low-level studies, flaws in the studies, etc.
• The available evidence is insufficient to assess effects on health and other outcomes of relevance to occupational therapy. Evidence is insufficient because of
  • The limited number or size of studies
  • Important flaws in study design or methods
  • Inconsistency of findings across individual studies
  • Lack of information on important health outcomes.

More information may allow estimation of effects on health and other outcomes of relevance to occupational therapy.