A reply

Abstract

We are grateful to Dr Butwick for spotting the error in our article 1, for which we apologise; we have corrected the bias and graphs accordingly in an Erratum 2. We also thank Dr Butwick for raising an important point about treating separate measurements on the same subjects as independent, in our Bland-Altman plots. There is no simple answer. Normally, not treating paired observations as dependent would lead to serious under-estimation of standard errors and misleading significance tests. However, significance testing is not used in the Bland-Altman method. Bland and Altman themselves point out that their approach ‘is based on the philosophy that the key to method comparison studies is to quantify disagreements between individual measurements. It follows that we do not see a place for methods of analysis based on hypothesis testing’ 3. Myles and Cui 4 suggest various methods of analysis for multiple measurements producing varying results, presumably with the purpose of avoiding estimated limits of agreement that appear more accurate than is actually the case. We feel that in this context, not accounting for dependency can be defended. We are not seeking to estimate the average difference or trend as accurately as possible while minimising the standard error. Instead, we are interested in describing the differences between the observations as accurately as possible. As such we are assured by the fact that Butwick and colleagues' findings are in accord with ours: there is a clinically unacceptable variation of the values obtained by the Masimo device, which renders it less useful in clinical practice.