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A Simple Guide to Effect Size Measures

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1 Credit CME

Importance  Effect size quantifies the magnitude of the difference or the strength of the association between variables. In clinical research it is important to calculate and report the effect size and the confidence interval (CI) because it is needed for sample size calculation, meaningful interpretation of results, and meta-analyses.

Observations  There are many different effect size measures that can be organized into 2 families or groups—d family and r family. The d family includes measures that quantify the differences between groups. The r family includes measures that quantify the strength of the association. Effect sizes that are presented in the same units as the characteristic being measured and compared are known as nonstandardized or simple effect sizes. The nonstandardized effect sizes have the advantage of being more informative, easier to interpret, and easier to evaluate in the light of clinical significance or practical relevance. Standardized effect sizes are unit-less and are helpful for combining and comparing effects of different outcome measures or across different studies (ie, meta-analysis).

Conclusions and Relevance  The choice of the correct effect size measure depends on the research question, study design, targeted audience, and the statistical assumptions being made. For a complete and meaningful interpretation of results from a clinical research study, the investigator should make clear the type of effect size being reported, its magnitude and direction, degree of uncertainty of the effect size estimate as presented by the CIs, and whether the results are compatible with a clinically meaningful effect.

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Article Information

Accepted for Publication: January 27, 2023.

Published Online: March 23, 2023. doi:10.1001/jamaoto.2023.0159

Corresponding Author: Dorina Kallogjeri, MD, MPH, Clinical Outcomes Research Office, Department of Otolaryngology–Head and Neck Surgery, Washington University School of Medicine in St Louis, 660 S Euclid Ave, Campus Box 8115, St Louis, MO 63110 (kallogjerid@wustl.edu).

Author Contributions: All authors had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: All authors.

Drafting of the manuscript: All authors.

Critical revision of the manuscript for important intellectual content: Kallogjeri.

Administrative, technical, or material support: Piccirillo.

Other - wrote the manuscript: Kallogjeri.

Conflict of Interest Disclosures: Dr Piccirillo reported that he developed the SNOT, SNORE, and NOSE-HHT instruments and receives royalty payments when Washington University licenses these instruments. No other conflicts were reported.

Disclaimer: Dr Piccirillo is Editior in Chief of JAMA Otolaryngology–Head and Neck Surgery. Dr Kallogjeri is the Statistics Editor of JAMA Otolaryngology–Head and Neck Surgery. They were not involved in the editorial decision regarding this manuscript.

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AMA CME Accreditation Information

Credit Designation Statement: The American Medical Association designates this Journal-based CME activity activity for a maximum of 1.00  AMA PRA Category 1 Credit(s)™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Successful completion of this CME activity, which includes participation in the evaluation component, enables the participant to earn up to:

  • 1.00 Medical Knowledge MOC points in the American Board of Internal Medicine's (ABIM) Maintenance of Certification (MOC) program;;
  • 1.00 Self-Assessment points in the American Board of Otolaryngology – Head and Neck Surgery’s (ABOHNS) Continuing Certification program;
  • 1.00 MOC points in the American Board of Pediatrics’ (ABP) Maintenance of Certification (MOC) program;
  • 1.00 Lifelong Learning points in the American Board of Pathology’s (ABPath) Continuing Certification program; and
  • 1.00 CME points in the American Board of Surgery’s (ABS) Continuing Certification program

It is the CME activity provider's responsibility to submit participant completion information to ACCME for the purpose of granting MOC credit.

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