= Plotting agreement between two measures using the Bland-Altman plot =
Two tests which purport to measure the same underlying variable may be highly correlated (form a straight line when plotted against each other) but not agree. Agreement would be indicated by both tests taking the same value with the line x=y representing the best fitting straight line in a scatterplot of the tests graphed against one another.
Bland and Altman (1995) suggested checking agreement between a pair of measures by plotting their difference on the y-axis against their sum on the x-axis. They also suggest working out the mean inter-test difference and the confidence interval for this difference (equal to mean difference +/- 1.96 SD of the differences) and adding these to the scatterplot. These statistics [[FAQ/balSPSS| can be worked out in SPSS]] although SPSS will not add the lines representing the limits of the confidence interval or the mean for the inter-test difference to the scatter plot. Further details with illustrations are given [[http://www.medcalc.org/manual/blandaltman.php|here]] and [[attachment:balt.pdf|here.]] If the scatterplot is random, the mean difference is around zero and the inter-test differences within +/-1.96 of the mean then the tests may be used interchangeably.
__Reference__
Bland JM, Altman DG (1995). Comparing methods of measurement: why plotting difference against standard method is misleading. ''The Lancet'' '''346''' 1085-1087.