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| [[Image:Bland-altman plot.png|thumb|300px|Bland-Altman plot]]
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| A '''Bland–Altman plot''' (Difference plot) in [[analytical chemistry]] and [[biostatistics]] is a method of [[data plot]]ting used in analyzing the agreement between two different [[assay]]s. It is identical to a '''[[John Tukey|Tukey]] mean-difference plot''', the name by which it is known in other fields, but was popularised in [[medical statistics]] by [[J. Martin Bland]] and [[Doug Altman|Douglas G. Altman]].<ref name="Altman1983">{{cite journal |author=Altman DG, Bland JM |title=Measurement in medicine: the analysis of method comparison studies |journal=The Statistician |volume=32 |year=1983 |pages=307–317 |doi=10.2307/2987937}}</ref><ref name="Bland1986">{{cite journal |author=Bland JM, Altman DG |title=Statistical methods for assessing agreement between two methods of clinical measurement |journal=Lancet |volume=327 | year=1986 |issue=8476 |pages=307–10 |doi=10.1016/S0140-6736(86)90837-8 |pmid=2868172}}</ref>
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| ==Agreement vs correlation==
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| Bland and Altman make the point that any two methods that are designed to measure the same parameter (or property) should have good [[correlation]] when a set of samples are chosen such that the property to be determined varies considerably. A high correlation for any two methods designed to measure the same property could thus in itself just be a sign that one has chosen a wide spread sample. A high correlation does not automatically imply that there is good agreement between the two methods.
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| ==How to construct a Bland-Altman plot==
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| Consider a set of ''n'' samples (for example, objects of unknown volume). Both assays (for example, different methods of volume measurement) are performed on each sample, resulting in 2''n'' data points. Each of the ''n'' samples is then represented on the graph by assigning the [[mean]] of the two measurements as the abscissa (x-axis) value, and the difference between the two values as the ordinate (y-axis) value.
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| Hence, the [[Cartesian coordinate system|Cartesian coordinates]] of a given sample ''S'' with values of <math>S_1</math> and <math>S_2</math> determined by the two assays is
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| :<math> S(x,y)=\left( \frac{S_1+S_2}{2},(S_1-S_2) \right). </math>
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| ==Application==
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| One primary application of the Bland–Altman plot is to compare two clinical measurements that each provide some errors in their measure.<ref name="Hanneman2008">{{cite journal |author=Hanneman SK |title=Design, analysis, and interpretation of method-comparison studies |journal=AACN Advanced Critical Care |volume=19 |year=2008 |pages=223–234 |pmid=18560291 |doi=10.1097/01.AACN.0000318125.41512.a3 |issue=2 |pmc=2944826}}</ref> It can also be used to compare a new measurement technique or method with a [[gold standard (test)|gold standard]] even so the interest of the Bland–Altman plot is contested in this particular case because the error pertains to the sole new measure. See [[Analyse-it]] or [[MedCalc]] for software providing Bland–Altman plots.
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| Bland and Altman plots are extensively used to evaluate the agreement among two different instruments or two measurements techniques. Bland and Altman plots allow us to investigate the existence of any systematic difference between the measurements (i.e., fixed bias) and to identify possible [[outlier]]s. The mean difference is the estimated bias, and the [[Standard deviation|SD]] of the differences measures the random fluctuations around this mean. If the mean value of the difference differs significantly from 0 on the basis of a 1-sample [[Student's t-test|t-test]], this indicates the presence of fixed bias. If there is a consistent bias, it can be adjusted for by subtracting the mean difference from the new method. It is common to compute 95% [[limits of agreement]] for each comparison (average difference ± 1.96 standard deviation of the difference), which tell us how far apart measurements by 2 methods were more likely to be for most individuals. If the differences within mean ± 1.96 SD are not clinically important, the two methods may be used interchangeably.
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| Bland and Altman plots were also used to investigate any possible relationship of the discrepancies between the measurements and the true value (i.e., proportional bias). The existence of proportional bias indicates that the methods do not agree equally through the range of measurements (i.e., the limits of agreement will depend on the actual measurement). To evaluate this relationship formally, the difference between the methods should be regressed on the average of the 2 methods. When a relationship between the differences and the true value was identified (i.e., a significant slope of the regression line), regression-based 95% limits of agreement should be provided.<ref name="Bland1999">{{cite journal |author=Altman DG, Bland JM Bland JM, Altman DG |title=Measuring agreement in method comparison studies |journal=[[Statistical Methods in Medical Research]] |volume=8 | year=1999|issue=2 |pages=135–60 |doi= 10.1191/096228099673819272|pmid=10501650| url =}}</ref>
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| ==Note==
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| A similar method was proposed in 1981 by Eksborg.<ref name=Eksborg1981>Eksborg S (1981) Evaluation of method-comparison data. Clin Chem 27:1311-1312</ref> This method was based on [[Deming regression]] - a method introduced by Adcock in 1878.
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| ==See also==
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| *[[MA plot]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://rstats.tiddlyspot.com/#Bland-Altman Bland-Altman plot explanation with R statistical programming language.]
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| * [http://analyse-it.com/products/method_evaluation/accuracy.aspx Bland-Altman plots using Analyse-it and Excel]
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| * [http://www.medcalc.org/manual/blandaltman.php Bland-Altman plots using MedCalc]
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| * [http://www.stattutorials.com/SAS/TUTORIAL-BLAND-ALTMAN.htm Bland-Altman Analysis using SAS]
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| * [http://ideas.repec.org/c/boc/bocode/s404501.html Bland-Altman analysis, with Lin's concordance coefficient, in Stata]
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| * [http://www-users.york.ac.uk/~mb55/meas/meas.htm Martin Bland's home page - Design and analysis of measurement studies]
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| {{DEFAULTSORT:Bland-Altman plot}}
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| [[Category:Analytical chemistry]]
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| [[Category:Statistical charts and diagrams]]
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| [[Category:Medical statistics]]
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Greetings. The author's title is Phebe and she feels comfortable when individuals use the complete title. My working day job is a meter reader. Minnesota is exactly where he's been living for many years. The favorite pastime for my kids and me is to play baseball but I haven't made a dime with it.
Feel free to visit my blog post; at home std test