Misuse of statistical terminology is unfortunately very common in medical research reports. The misuse not only indicate methodological ignorance, it also threatens the consistency of the statistical terminology. For example, Nahoui et al. (1) state that in their sample of patients, those "in 3rd and 4th quartiles of median PES [esophageal pressure] had increased mortality risk compared to 1st quartile". Given that only three quartiles exist, the statement is remarkable.
The correct definition (2) of quartile is: "There are three values which separate the total frequency of a distribution into four equal parts. The central value is called the median and the other two the lower (first) and upper (third) quartiles respectively."
The authors obviously confuse quartiles with the four quarts of the distribution defined by the three quartiles. Similar misunderstandings of other quantiles, such as tertiles and quintiles, are as frequent. Furthermore, the misunderstanding of the term quartiles have implications for the interpretation of a widely used measure of dispersion: The interquartile range (IQR), defined as the difference between the upper and lower quartiles. This measure shows the spread of half of the total frequency.
In addition, note that the range (the largest minus the smallest of a set of variable values) and the interquartile range are one value each, not the two quartiles or values that are used to calculate them, as is frequently the case in medical research reports.
References
1. Nahoui H, Schirmer H, Einvik G, Øvrebotten T, Berge K, Myhre PL, Husby H, Hrubos-Strøm H. Respiratory effort during sleep predicts mortality in patients with suspected obstructive sleep apnea. Am J Respir Crit Care Med. 2026 Jun 29:aamag344. doi: 10.1093/ajrccm/aamag344. Epub ahead of print. PMID: 42371748.
2. International Statistical Institute. The Oxford Dictionary of Statistical Terms. Sixth Edition. Oxford, New York: Oxford University Press; 2006.
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