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ARTICLE

The Effect of Clustering of Outcomes on the Association of Procedure Volume and Surgical Outcomes

Katherine S. Panageas, DrPH; Deborah Schrag, MD, MPH; Elyn Riedel, MA; Peter B. Bach, MD, MAPP; and Colin B. Begg, PhD

21 October 2003 | Volume 139 Issue 8 | Pages 658-665

Background: A large body of literature documents associations between the volume of cases a hospital or surgeon treats and clinical outcomes. Most of these studies have used conventional statistical methods that do not recognize the fact that hospitals or surgeons with similar volumes may have very different outcomes because of systematic differences in processes of care, a phenomenon that exaggerates the true statistical significance of the effect of volume on outcome.

Objective: To describe methods to assess the degree of this "clustering" of outcomes and to explore the impact of available statistical techniques that correct for clustering.

Design: Reanalysis of 3 previously published volume–outcome studies.

Setting: Medicare beneficiaries 65 years of age or older undergoing surgery for colon, prostate, or rectal cancer in the population defined by the Surveillance, Epidemiology, and End Results cancer registries during 1992 to 1996.

Patients: 3 data sets were analyzed to assess the impact of surgeon volume on outcomes: 1) 24 166 colectomies performed by 2682 surgeons, 2) 10 737 prostatectomies performed by 999 surgeons, and 3) 2603 rectal resections performed by 1141 surgeons.

Measurements: Volume–outcome trends were analyzed by a conventional method (logistic regression) and corrected for clustering. Two widely used statistical methods for analyzing clustered data, a random-effects model and generalized estimating equations, were used and compared, and the degree of clustering was presented graphically.

Results: Substantial clustering was observed in the analyses involving morbidity end points. The 2 statistical techniques produced noticeably different results in some analyses.

Conclusions: The presence of clustering represents variations in outcomes among providers with similar volumes. Thus, in volume–outcome studies, the degree of clustering of outcomes should be characterized because it may provide insight into variations in quality of care.

Editors' Notes Context While studies suggest that surgical patients fare best with providers (hospitals and surgeons) that perform a high volume of procedures, most have not accounted for the tendency of patients of 1 provider to have similar outcomes ("clustering"). When outcomes of individual patients are clustered, more patients are required to prove that providers' outcomes differ from one another. Contribution This reanalysis of data from 3 previously published volume–outcome studies of surgery accounted for clustering within provider. Statistical significance of volume–outcome relationships of morbidity end points was attenuated substantially after adjustment for the effects of clustering. Implications Planners considering regionalizing surgery should remember that volume–outcome studies that have not accounted for clustering exaggerate differences in outcomes by provider. –The Editors

 

Author and Article Information

From Memorial Sloan-Kettering Cancer Center, New York, New York.

Acknowledgments: The authors thank 2 anonymous reviewers for help with the article. They also thank the groups responsible for the creation and dissemination of the linked database, including the Applied Research Branch, Division of Cancer Control and Population Sciences, National Cancer Institute; the Office of Strategic Planning and the Office of Informational Services, Centers for Medicare & Medicaid Services; Information Management Services; and the Surveillance, Epidemiology, and End Results tumor registries.

Grant Support: In part by grants from the National Cancer Institute (CA83950 [Dr. Schrag], CA90226 [Dr. Bach], and CA08748 [Dr. Begg]).

Potential Financial Conflicts of Interest: None disclosed.

Requests for Single Reprints: Colin B. Begg, PhD, Memorial Sloan-Kettering Cancer Center, 307 East 63rd Street (3rd Floor), New York, NY 10021; e-mail, beggc{at}mskcc.org.

Current Author Addresses: Drs. Panageas, Schrag, Bach, and Begg and Ms. Riedel: Memorial Sloan-Kettering Cancer Center, 307 East 63rd Street (3rd Floor), New York, NY 10021.

Author Contributions: Conception and design: K.S. Panageas, D. Schrag, E. Riedel, C.B. Begg.

Analysis and interpretation of the data: K.S. Panageas, D. Schrag, E. Riedel, C.B. Begg.

Drafting of the article: K.S. Panageas, D. Schrag, C.B. Begg.

Critical revision of the article for important intellectual content: K.S. Panageas, D. Schrag, P.B. Bach, C.B. Begg.

Final approval of the article: K.S. Panageas, P.B. Bach, C.B. Begg.

Provision of study materials or patients: D. Schrag, C.B. Begg.

Statistical expertise: K.S. Panageas, E. Riedel, C.B. Begg.

Obtaining of funding: C.B. Begg.

Administrative, technical, or logistic support: C.B. Begg.

Collection and assembly of data: D. Schrag, C.B. Begg.

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