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!
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