A constant within subject standard deviation cannot be assumed a priori

Editor–In their statistical note about measurement error J Martin Bland and Douglas G Altman state that there is no point in estimating within subject standard deviation if we cannot assume that it is the same for all subjects.1 This assumption, however, does not hold true in every clinically relevant situation. For example, the within subject standard deviation of many analyses is increased in patients from intensive care units. Even outside these extreme conditions, a constant within subject standard deviation cannot be assumed a priori. Homogeneity of within subject standard deviation must be verified before the estimate obtained from analysis of variance can be applied to determine whether two consecutive results in a single patient are truly different.2

When within subject standard deviation is not constant from one patient to another (presence of heterogeneity) the use of a single “mean” within subject standard deviation to estimate repeatability will underestimate the variability for several patients and will possibly lead to false conclusions about the clinical importance of the …