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      Understanding Bland Altman analysis

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          Abstract

          In a contemporary clinical laboratory it is very common to have to assess the agreement between two quantitative methods of measurement. The correct statistical approach to assess this degree of agreement is not obvious. Correlation and regression studies are frequently proposed. However, correlation studies the relationship between one variable and another, not the differences, and it is not recommended as a method for assessing the comparability between methods.
In 1983 Altman and Bland (B&A) proposed an alternative analysis, based on the quantification of the agreement between two quantitative measurements by studying the mean difference and constructing limits of agreement.
The B&A plot analysis is a simple way to evaluate a bias between the mean differences, and to estimate an agreement interval, within which 95% of the differences of the second method, compared to the first one, fall. Data can be analyzed both as unit differences plot and as percentage differences plot.
The B&A plot method only defines the intervals of agreements, it does not say whether those limits are acceptable or not. Acceptable limits must be defined a priori, based on clinical necessity, biological considerations or other goals.
The aim of this article is to provide guidance on the use and interpretation of Bland Altman analysis in method comparison studies.

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          Most cited references 10

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          Statistical methods for assessing agreement between two methods of clinical measurement.

          In clinical measurement comparison of a new measurement technique with an established one is often needed to see whether they agree sufficiently for the new to replace the old. Such investigations are often analysed inappropriately, notably by using correlation coefficients. The use of correlation is misleading. An alternative approach, based on graphical techniques and simple calculations, is described, together with the relation between this analysis and the assessment of repeatability.
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            Comparing methods of measurement: why plotting difference against standard method is misleading.

            When comparing a new method of measurement with a standard method, one of the things we want to know is whether the difference between the measurements by the two methods is related to the magnitude of the measurement. A plot of the difference against the standard measurement is sometimes suggested, but this will always appear to show a relation between difference and magnitude when there is none. A plot of the difference against the average of the standard and new measurements is unlikely to mislead in this way. We show this theoretically and by a practical example.
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              Why Bland-Altman plots should use X, not (Y+X)/2 when X is a reference method.

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                Author and article information

                Journal
                Biochem Med (Zagreb)
                Biochem Med (Zagreb)
                BM
                Biochemia Medica
                Croatian Society of Medical Biochemistry and Laboratory Medicine
                1330-0962
                1846-7482
                05 June 2015
                June 2015
                : 25
                : 2
                : 141-151
                Affiliations
                Clinical Chemistry and Hematology Laboratory, San Bortolo Hospital, Vicenza, Italy
                Author notes
                Article
                bm-25-141
                10.11613/BM.2015.015
                4470095
                Categories
                Review Article

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