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Validation of Statistical Methods to Compare Cancellation Rates on the Day of Surgery


Dexter, Franklin MD, PhD; Shafer, Steven L. MD

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doi: 10.1213/ANE.0b013e31824e8a98
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The appendix of a 2005 paper gives steps for iteratively calculating the inverse of the Freeman-Tukey transform.1 Step 1 should have used the harmonic mean sample size among 4-week periods, not the total sample size.2 Differences in cancellation rates will be small (e.g., < 0.1% using the numbers in Table 8 in the article), because sample sizes are large for benchmarking cancellations (N > 100 cases).1 The correction has no effect on the paper's results and example (i.e., Tables 1–8 in the article are identical), because all calculations with the transform were performed in the transformed space.1 The correction also has no effect on the results of the subsequent cancellation paper,3 because the analyses were performed without transformation. The inverse transform can readily be calculated as described by Miller.2


1. Dexter F, Marcon E, Epstein RH, Ledolter J. Validation of statistical methods to compare cancellation rates on the day of surgery. Anesth Analg 2005;101:465–73
2. Miller JJ. The inverse of the Freeman-Tukey double arcsine transformation. Am Stat 1978;32:138
3. Tung A, Dexter F, Jakubczyk S, Glick DB. The limited value of sequencing cases based on their probability of cancellation. Anesth Analg 2010;111:749–56
© 2012 International Anesthesia Research Society