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Bland-Altman plot with multiple measurements per subject

Command:Statistics
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Description

Create a Bland-Altman plot for method comparison when there is more than one measurement per subject with each laboratory method.

The Bland-Altman plot (Bland & Altman, 1986, 1999, 2007), or difference plot, is a graphical method to compare two measurements techniques. In this graphical method the differences (or alternatively the ratios) between the two techniques are plotted against the averages of the two techniques. Alternatively (Krouwer, 2008) the differences can be plotted against one of the two methods, if this method is a reference or "gold standard" method.

Horizontal lines are drawn at the mean difference, and at the limits of agreement, which are defined as the mean difference plus and minus 1.96 times the standard deviation of the differences. If the differences within mean ± 1.96 SD are not clinically important, the two methods may be used interchangeably.

The plot is useful to reveal a relationship between the differences and the averages, to look for any systematic biases and to identify possible outliers.

How to enter data

This procedure requires that you have your data organized like in the following example (data from Bland & Altman, 2007):

How to enter data for Bland-Altman plot with multiple measurements per subject

There is one column for subject identification (Subject) and one column for the measurements for each method (RV and IC).

If you have your data organized in a different format, such as the data for the multiple measurements in different columns, you can use the Stack columns tool to reorganize your data (see also the Stack columns worked example).

Required input

Dialog box for Bland-Altman plot with multiple measurements per subject

Data

Model

Options

Calculations

The calculations are performed as described by Zou (2013). For the estimation of the confidence intervals of the limits of agreement, the MOVER method is used (Zou, 2013).

Results

MedCalc creates a graph and a report.

Graph

This is the graph in the True value is constant in each subject model:

Bland-Altman plot with multiple measurements per subject - true value is constant

In the True value is constant in each subject model (see Bland & Altman, 2007) there is only one marker for each subject in the graph, and the marker size is relative to the number of observations for the subject. The number of markers is equal to the number of subjects.

In the alternative model, where the True value varies, there is one marker for each observation pair:

Bland-Altman plot with multiple measurements per subject - true value varies

Confidence intervals

Optionally, confidence intervals may be displayed for the average difference and for the limits of agreement. These confidence intervals can be represented as error bars or horizontal lines. Right-click on the error bar to set formatting options.

Report

The report contains the exact values for average difference and the limits of agreement with their confidence intervals.

Bland-Altman plot with multiple measurements per subject - report
Bland-Altman plot with multiple measurements per subject

Method A

RV

Method B

IC

Sample size

120

Model

The true value is constant in each subject

Option

Plot differences

Arithmetic mean

0.7092

Lower limit

-1.3524

95% Confidence interval

-2.6992 to -0.6284

Upper limit

2.7709

95% Confidence interval

2.0468 to 4.1177

Literature

See also