• Andrew Carkeet.
• The online, full-text version of this article might not render algebraic expression correctly. Readers may wish to download the full-text, PDF version of this article, which displays the expressions clearly and accurately. *BAppSciOptom(Hons), PhD School of Optometry and Vision Science, Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia.
• Optom Vis Sci. 2015 Mar 1;92(3):e71-80.

PurposeThe previous literature on Bland-Altman analysis only describes approximate methods for calculating confidence intervals for 95% limits of agreement (LoAs). This article describes exact methods for calculating such confidence intervals based on the assumption that differences in measurement pairs are normally distributed.MethodsTwo basic situations are considered for calculating LoA confidence intervals: the first, where LoAs are considered individually (i.e., using one-sided tolerance factors for a normal distribution); and the second, where LoAs are considered as a pair (i.e., using two-sided tolerance factors for a normal distribution). Equations underlying the calculation of exact confidence limits are briefly outlined.ResultsTo assist in determining confidence intervals for LoAs (considered individually and as a pair), tables of coefficients have been included for degrees of freedom between 1 and 1000. Numerical examples, showing the use of the tables for calculating confidence limits for Bland-Altman LoAs, have been provided.ConclusionsExact confidence intervals for LoAs can differ considerably from the Bland and Altman approximate method, especially for sample sizes that are not large. There are better, more precise methods for calculating confidence intervals for LoAs than the Bland and Altman approximate method, although even an approximate calculation of confidence intervals for LoAs is likely to be better than none at all. Reporting confidence limits for LoAs considered as a pair is appropriate for most situations; however, there may be circumstances where it is appropriate to report confidence limits for LoAs considered individually.

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