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Research Article  |   March 1999
How To Detect Effects: Statistical Power and Evidence-Based Practice in Occupational Therapy Research
Author Affiliations
  • Kenneth J. Ottenbacher, PhD, OTR, FAOTA, is Vice Dean, School of Allied Health Sciences, University of Texas Medical Branch, 301 University Boulevard, Galveston, Texas 77555-1028, kottenba@utmb.edu
  • Frikkie Maas, PhD, is Senior Lecturer, Department of Occupational Therapy, School of Health and Rehabilitation Science, University of Queensland, Australia
Article Information
Evidence-Based Practice / Research Methodology / Quantitative Research Series
Research Article   |   March 1999
How To Detect Effects: Statistical Power and Evidence-Based Practice in Occupational Therapy Research
American Journal of Occupational Therapy, March/April 1999, Vol. 53, 181-188. doi:10.5014/ajot.53.2.181
American Journal of Occupational Therapy, March/April 1999, Vol. 53, 181-188. doi:10.5014/ajot.53.2.181
Abstract

The findings from 30 research investigations examining the effectiveness of occupational therapy interventions were reviewed and analyzed. The statistical conclusion validity was determined by computing post hoc power coefficients for the statistical hypothesis tests included in the examined studies. Data analysis revealed the median power values to detect small, medium, and large effect sizes were .09, .33, and .66, respectively. These results suggest a high probability of Type II errors in the sample of occupational therapy intervention research examined. In practical terms, this means the intervention produced a potentially useful treatment effect, but the effect was not detected as significant. Examples are provided that illustrate how low statistical power contributes to increases in Type II errors and inhibits the development of consensus through replication in the research literature. The presence of low-power studies with high rates of false negative findings prevents the establishment of guidelines for evidence-based practice and impedes the scientific progress of rehabilitation professions such as occupational therapy.