Department of Anesthesiology and General Intensive Care, University of Vienna, Austria and Department of Anesthesiology, Washington University School of Medicine, St Louis, MO, USA*Address for correspondence: Department of Anesthesiology and General Intensive Care, University of Vienna, Währinger Gürtel 1820, A-1090 Vienna, Austria. E-mail: peter.nagele@univie.ac.at
Accepted for publication: December 3, 2002
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Abstract |
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Methods. The aim of this study was to evaluate the frequency of inappropriate use of the SEM in four leading anaesthesia journals in 2001. The journals were searched manually for descriptive statistics reporting either the mean (SD) or the mean (SEM), and inappropriate use of the SEM was noted.
Results. In 2001, all four anaesthesia journals published articles that used the SEM incorrectly: Anesthesia & Analgesia 27.7%, British Journal of Anaesthesia 22.6%, Anesthesiology 18.7% and European Journal of Anaesthesiology 11.5%. Laboratory reports and clinical studies were equally affected, except for Anesthesiology where 90% were basic science reports.
Conclusions. One in four articles (n=198/860, 23%) published in four anaesthesia journals in 2001 inappropriately used the SEM in descriptive statistics to describe the variability of the study sample. Anaesthesia journals are encouraged to provide clearer statistical guidelines on how to report data variability in descriptive statistics.
Br J Anaesth 2003; 90: 51416
Keywords: statistics
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Introduction |
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If normally distributed, the study sample can be described entirely by two parameters: the mean and the standard deviation (SD). The SD represents the variability within the sample; the larger the SD, the higher the variability within the sample.1 Although it is clear that samples should always be summarized by the mean and SD,25 authors often use the standard error of the mean (SEM) to describe the variability of their sample. The SEM is used in inferential statistics to give an estimate of how the mean of the sample is related to the mean of the underlying population. As the SEM is always smaller than the SD, the unsuspecting reader may think that the variability within the sample is much smaller than it really is. Although the SD and the SEM are related (SEM=SD/n), they give two very different types of information.6 Whereas the SD estimates the variability in the study sample, the SEM estimates the precision and uncertainty of how the study sample represents the underlying population.1 7 In other words, the SD tells us the distribution of individual data points around the mean, and the SEM informs us how precise our estimate of the mean is.3 It is therefore inappropriate and incorrect to present data only as the mean (SEM).
This evaluation was designed to identify the frequency of this statistical error in articles published in 2001 in four leading anaesthesia journals: two from the USA (Anesthesiology and Anesthesia & Analgesia), and two from Europe (British Journal of Anaesthesia and European Journal of Anaesthesiology).
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Methods and results |
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A total of 257 articles fulfilled the search criteria in Anesthesiology, 405 articles in Anesthesia & Analgesia, 137 in the British Journal of Anaesthesia, and 61 in the European Journal of Anaesthesiology. Detailed results are given in Table 1, where the four journals are listed in order of decreasing percentage misuse of SEM. Eight articles each in Anesthesiology and Anesthesia & Analgesia even failed to state which parameter was used. It must be noted that in some of the articles that incorrectly used the SEM, both parameters, SEM and SD, were used. In these articles, the SD was mostly found in the text and the SEM in the figures.
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Discussion |
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In conclusion, in spite of clear recommendations, the SEM is still widely and inappropriately used in the anaesthesia literature. Anaesthesia journals could easily avoid this statistical error by requiring authors to adhere to statistical recommendations, for instance through a more stringent statistical review process. The goal should be to have one standard method to describe the distribution of a study sample, thereby reducing confusion among the readers of biomedical research papers.
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Acknowledgements |
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The author was supported by the Fonds zur Förderung der Wissenschaftlichen Forschung (FWF)Austrian Science Fundas the recipient of an Erwin-Schrödinger research fellowship.
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References |
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