# Bland Altman plot

**Description**

The Bland-Altman plot is a graphical comparison of two different measurements (or techniques) of a continuously scaled variable. The Bland-Altman forms a scatterplot with the difference of the two measurements for each sample on the vertical axis and the average of the two measurements on the horizontal axis.

Three horizontal reference lines are superimposed on the scatterplot, one denoting average difference with the other lines forming some form of upper and lower bounds. These are usually the standard deviation of measurement differences but can also be 95% confidence interval.

**Worked Example**

Download example excel file provided below

__Instructions__

- Prepare your data in the format of the sample data. Test A and Test B are in columns with individual values below.
- Click on Analyse below and upload your .csv or .xlsx data file (indicate which you are uploading), the example provided is .xlsx
- Select the tests you would like to prepare in ‘Select variables’
- You will then see the confirmed data in ‘Selected columns’
- The statistics tab gives exact values for sigma level as well as confidence intervals.
- Bland Altman plot-1 tab gives the mean difference and sigma bounds whilst confidence interval gives the upper and lower bounds as confidence intervals

Written By Daniel Richardson