Intended for healthcare professionals

Rapid response to:

Endgames Statistical Question

Normal ranges

BMJ 2013; 346 doi: https://doi.org/10.1136/bmj.f1343 (Published 08 March 2013) Cite this as: BMJ 2013;346:f1343

Rapid Response:

Re: Normal ranges

Dr. Sedgwick has written several Endgames (Normal Ranges; Standard deviation vs. standard error; Limits of agreement (Bland-Altman method)) which contain the statement that normal ranges of +/- one, two or three standard deviations can be calculated for any continuous variable, whether it follows a Normal distribution or not. This is misleading.

Of course, one may calculated whatever one chooses; but Dr. Sedgwick writes that these 'three ranges are typically derived that contain about 68%, 95% and 99% of the sample participants'. This will not always hold if the variable is not (approximately) normally distributed.

I would not have quibbled about this matter if it were not for the fact that Dr. Sedgwick includes this issue explicitly in two of the multiple choice questions (Normal ranges: statement c and Limits of agreement: statement c). In my opinion, the suggested answer is incorrect - or at least misleading!

Competing interests: No competing interests

18 February 2014
Jeremy Franklin
medical statistician
University of Cologne, Hospital
Kerpener Str. 62,