Bland-Altman plot

In analytical chemistry and biostatistics, a Bland-Altman plot is a method of data plotting used in analysing the agreement between two different assays. It is identical to a Tukey mean-difference plot, which is what it is still known as in other fields, but was popularised in medical statistics by Bland and Altman.cite journal |author=Altman DG, Bland JM |title=Measurement in medicine: the analysis of method comparison studies |journal=Statistician |volume=32 |year=1983 |pages=307–317 |doi=10.2307/2987937] cite journal |author=Bland JM, Altman DG |title=Statistical methods for assessing agreement between two methods of clinical measurement |journal=Lancet |volume=1 | year=1986 |issue=8476 |pages=307–10 |pmid=2868172]

Agreement vs correlation

Bland and Altman make the point that any two methods that are designed to measure the same parameter (or property) will have a good correlation when a set of samples are chosen such that the property to be determined vary a lot between them. A high correlation for any two methods designed to measure the same property is thus in itself is just 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.

How to construct a Bland-Altman plot

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.

Hence, the Cartesian coordinates of a given sample "S" with values of $S\_1$ and $S\_2$ determined by the two assays is

$S(x,y)=left(\; frac\{S\_1+S\_2\}\{2\},(S\_1-S\_2)\; ight)$

Application

One primary application of the Bland-Altman plot is to compare two clinical measurements that each provide some errors in their measure.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 ] It can also be used to compare a new measurement technique or method with a 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.

References

External links

* [http://rstats.tiddlyspot.com/#Bland-Altman Bland-Altman plot explanation with R statistical programming language.]
* [http://analyse-it.com/products/method_evaluation/accuracy.aspx Bland-Altman plots using Analyse-it and Excel]
* [http://www.stattutorials.com/SAS/TUTORIAL-BLAND-ALTMAN.htm Bland-Altman Analysis using SAS]