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- Published on: 3 August 2022
- Published on: 3 August 2022Turnover times on the same day are not statistically independent
Riveros Perez et al. analysed 636 turnover times from a surgical suite with 16 theatres before and after intervention, a dedicated nurse anaesthetist for each of four theatres [1]. Their “overall” statistical analysis, reported in their abstract, seems to treat all turnovers as statistically independent events (i.e., treated the sample size as 16 x # analysed days). That probably was incorrect because the 636 turnover times likely were correlated among theatres on the same day [2]. The authors’ Wilcoxon-Mann-Whitney overall P-value (0.0121) likely is an underestimate of the correct result [2].
To understand, consider that the authors’ intervention was one extra nurse anaesthetist for four theatres. If there were more than one turnover among the four theatres simultaneously, the nurse anaesthetist’s efforts would be diluted. That is precisely what happens routinely (e.g., for housekeeping staff [3,4]). Earlier, we showed validity and reliability of choosing the optimal number of shared personnel (e.g., nurse anaesthetist) by analysing those simultaneous turnovers [3].
The authors state in their paper that the turnover times were skewed. Analysis methods tested by Monte-Carlo simulation for accurate P-values and confidence intervals are to take the mean of the turnovers among the 16 theatres, for each day, although the median could be used [2]. There then is one number per day as a summary measure. By central limit theorem (and in practice [2,5]), those means g...
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None declared.