Parts of a Bland-Altman Plot

1. The basic construction of a plot is that it contains two variables of which graph is to be plotted. The X-axis contains the mean of the two variables, and the Y-axis contains the difference between the variables. The X and Y values obtained are plotted.
2. An additional factor name CI(confidence interval) is also taken into consideration. The values are plotted for a 95% confidence interval.
3. A bias line is also created. Bias is the mean of the difference of variables. The line is parallel to the X-axis.
4. Line of Agreement(loA) is also created. Two loA are created in the Bland-Altman plot, parallel to the X-axis, which specifies the region of 95% confidence interval. The formula-driven to calculate it is +1.96S.D. and -1.96S.D., where S.D. is the standard deviation of the difference of the variables. 

Bland-Altman plot is not present by default in excel, but it can be easily built with the help of excel. There is software that provides in-built Bland-Altman plots like MedCalc, Analyse-it, etc. In this article, we will learn how to create a Bland-Altman plot in excel. A Bland-Altman plot is used to give agreement between two measured quantities or equipment. This plot is generally used in the medical industry to test the agreement and correlation values between two equipment or units. This plot was introduced in medicine statistics by J. Martin Bland and Douglas G. Altman. Bland and Altman said that in medical statistics, if two types of equipment are highly correlated, then it might not imply good agreement, as there could be widespread sample collection which leads to good correlation. To solve this issue, Bland and Altman introduced this plot, which gives you a qualitative analysis of whether the equipment or units used are highly correlated or not. The plot does not give a probability of error on the variables.