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Monte Carlo Simulations Comparing Fisher Exact Test and Unequal Variances t Test for Analysis of Differences Between Groups in Brief Hospital Lengths of Stay

Dexter, Franklin MD, PhD*; Bayman, Emine O. PhD; Dexter, Elisabeth U. MD, FACS

doi: 10.1213/ANE.0000000000002428
General Articles: Original Clinical Research Report

BACKGROUND: We examined type I and II error rates for analysis of (1) mean hospital length of stay (LOS) versus (2) percentage of hospital LOS that are overnight. These 2 end points are suitable for when LOS is treated as a secondary economic end point.

METHODS: We repeatedly resampled LOS for 5052 discharges of thoracoscopic wedge resections and lung lobectomy at 26 hospitals.

RESULTS: Unequal variances t test (Welch method) and Fisher exact test both were conservative (ie, type I error rate less than nominal level). The Wilcoxon rank sum test was included as a comparator; the type I error rates did not differ from the nominal level of 0.05 or 0.01. Fisher exact test was more powerful than the unequal variances t test at detecting differences among hospitals; estimated odds ratio for obtaining P < .05 with Fisher exact test versus unequal variances t test = 1.94, with 95% confidence interval, 1.31–3.01. Fisher exact test and Wilcoxon-Mann-Whitney had comparable statistical power in terms of differentiating LOS between hospitals.

CONCLUSIONS: For studies with LOS to be used as a secondary end point of economic interest, there is currently considerable interest in the planned analysis being for the percentage of patients suitable for ambulatory surgery (ie, hospital LOS equals 0 or 1 midnight). Our results show that there need not be a loss of statistical power when groups are compared using this binary end point, as compared with either Welch method or Wilcoxon rank sum test.

Supplemental Digital Content is available in the text.Published ahead of print September 28, 2017.

From the *Division of Management Consulting, Department of Anesthesia, and Departments of Anesthesia and Biostatistics, University of Iowa, Iowa City, Iowa; and Department of Thoracic Surgery, Roswell Park Cancer Institute, Buffalo, New York.

Published ahead of print September 28, 2017.

Accepted for publication July 25, 2017.

Funding: Departmental.

The authors declare no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Reprints will not be available from the authors.

Address correspondence to Franklin Dexter, MD, PhD, Division of Management Consulting, Department of Anesthesia, University of Iowa, 200 Hawkins Dr, 6-JCP, Iowa City, IA 52242. Address e-mail to

© 2017 International Anesthesia Research Society
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