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Statistics
Tutorial
This
is one in a series of tutorials using examples from WINKS SDA Software
Bland-Altman Plots
A Bland-Altman analysis is a way to assess agreement between two
methods of clinical measurement. Using an example from a paper by Bland and
Altman (1986), suppose there is a measurement (or peak expiratory flow rate/PEFR)
made from using “Large” meter and a “Mini” meter. You want to know if they
measure the same thing.
Two plots are used to analyze this data, a plot of identity and a Bland-Altman plot. The data follows:
SUBJECT LARGE MINI
1 494.0 512.0
2 395.0 430.0
3 516.0 520.0
4 434.0 428.0
5 476.0 500.0
6 557.0 600.0
7 413.0 364.0
8 442.0 380.0
9 650.0 658.0
10 433.0 445.0
11 417.0 432.0
12 656.0 626.0
13 267.0 260.0
14 478.0 477.0
15 178.0 259.0
16 423.0 350.0
17 427.0 451.0
The first is called a plot of identity. It is similar to a regression plot, except the line in the plot is a line based on X = Y. If the scatter of points in this plot lies near the line, it indicates that the two ways of measuring PEFR are similar. To display this plot:
Step 1: Open BAEXAMP.DBF. Select Analyze/Advanced Regression and Comparisons/Bland Altman/Line of Equality Plot.
Step 2: Select the two variables LARGE and MINI. The following plot is displayed:
From this plot of identity, you can see that the scatter of points falls close to the line.
A second method of looking at this data is with a Bland-Altman Plot. This plots the average of the two values by subject against the difference. 95% confidence limits are also calculated and displayed on the graph (Mean difference ± 2 * Standard Deviation)
To display this plot (after opening the BAEXAMP DATA):
Step 1: Select Analyze/Advanced Regression and Comparisons/Bland Altman/Bland-Altman Plot.
Step 2: Select the two variables LARGE and MINI. The following plot is displayed:
If the two measures are measuring the same thing in a unbiased way, this plot should show a random scatter of points between the upper and lower confidence limits. The plot above is indicative of a good fit. If there is a pattern to the points, there might be some biased associated with the measurements.
This procedure is available in the WINKS Professional edition.
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