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Confounding and the Analysis of Multiple Variables in Hospital Epidemiology

Published online by Cambridge University Press:  02 January 2015

Jonathan Freeman ,
Donald A. Goldmann  and
John E. McGowan Jr.
Jonathan Freeman*
Affiliation:
Hospital Epidemiology Unit, Charming Laboratory, Department of Medicine, Brigham and Women's Hospital, andWest Roxbury VA Hospital, Boston, Massachusetts
Donald A. Goldmann
Affiliation:
Division of Infections Diseases, Children's Hospital, Harvard Medical School, Boston, Massachusetts
John E. McGowan Jr.
Affiliation:
Departments of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, Georgia
*
Channing Laboratory, 180 Longwood Avenue, Boston, MA 02115
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Abstract

Most information in hospital epidemiology comes from observational studies of hospitalized patients rather than planned experiments, and in such observational studies the characteristics of study patients may vary widely, even within a single hospital. Any comparison between hospital populations will usually contain additional, unintended contrasts among patients with varying degrees of health. Adult patients, for example, may have vastly different underlying diseases, and infants may be of substantially different birth weights. We used both underlying disease and birth weight as indices of the basic severity of illness in order to adjust for confounding by differences in underlying disease in reanalyses of several published studies. We give an example in which differing birth weights among groups of infants compared artifactually double the apparent effect of nosocomial infections as a cause of mortality, and another example in which differing degrees of severity of underlying illness artifactually halve the apparent effect of appropriate antibiotics in preventing death from bacteremia with gram-negative bacilli. We describe simple intuitive methods based on stratification, adapted from chronic disease epidemiology, to remove confounding effects during analyses.

Type
Original Articles
Information
Infection Control & Hospital Epidemiology , Volume 8 , Issue 11 , November 1987 , pp. 465 - 473
Copyright
Copyright © The Society for Healthcare Epidemiology of America 1987

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