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Physical Therapist Assistant

Levels of Evidence Hierarchy

For clinical questions, articles with the highest quality of evidence are best. The pyramid represents the Levels of Evidence Hierarchy. Studies are assigned to a level based on their design and strength of recommendation.


                            Source: Wood, G. L, & Haber, J. (2014). Nursing research: Methods and critical appraisal of evidence-based practice (8ed). St. Louis: Mosby/Elsevier

Types of studies

Level I

  • Clinical Practice Guideline: Recommendations for clinicians based on a systematic review of evidence.
  • Meta-Analysis: Combines the results of studies using quantitative methods to statistically estimate the effects of proposed interventions.
  • Systematic Review: Appraised summary of all experimental or quantitative research addressing a specific clinical question.

Level II

  • Randomized Controlled Trials: A randomized group of patients are assigned to an experimental group and a control group. These groups are followed up on for the variables/outcomes of interest.

Level III

  • Quasi-experimental Trial: Controlled trial with a nonrandomized group of patients. 

Level IV

  • Cohort Study: Identifies two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not. Follows the cohorts forward for the outcome of interest.
  • Case-Control Study: Identifies patients who have the outcome of interest (cases) and control patients without the same outcome and looks for exposure of interest.

Level V

  • Systematic Review: Appraised summary of all qualitative or descriptive research that addresses a specific clinical question.

Level VI

  • Case Report: Evidence form a single descriptive or qualitative study

Level VII

  • Expert Opinion: Descriptive study of a single case or series of cases with no research design or generalized information found in textbooks, encyclopedias, etc. 

Types of studies chart

Sources consulted: Jewell, D. V. (2011). Guide to evidence-based physical therapist practice (2nd ed.). Sudbury, MA: Jones & Bartlett Learning. ; Woolf, S. H., Grol, R., Hutchinson, A., Eccles, M., & Grimshaw, J. (1999). Potential benefits, limitations, and harms of clinical guidelines. BMJ : British Medical Journal, 318(7182), 527–530. ; Koffel, J. (2016). Understanding research study designs. Retrieved September 12, 2016, from