Bland-Altman Plot and Analysis
Check whether two methods of measuring the same thing agree closely enough to be used interchangeably.
Updated · By Bibin XavierData 60 × 3
| Subjecttext · 60 levels | Cuff_Devicenumeric | Wrist_Devicenumeric |
|---|---|---|
| S01 | 124 | 128 |
| S02 | 115 | 115 |
| S03 | 123 | 130 |
| S04 | 104 | 100 |
| S05 | 147 | 151 |
| S06 | 105 | 106 |
| S07 | 145 | 150 |
| S08 | 131 | 138 |
Variables and options
Results
60 rows · example dataAgreement statistics
| Statistic | Estimate | 95% CI |
|---|---|---|
| Bias (mean difference) | -2.03 | [-2.87, -1.20] |
| Lower limit (bias − 1.96 SD) | -8.36 | [-9.80, -6.91] |
| Upper limit (bias + 1.96 SD) | 4.29 | [2.85, 5.73] |
| Bias test (one-sample t) | t(59) = -4.88 | < .001 |
| Proportional bias (slope of difference on mean) | -0.052 | p = .033 |
Bland-Altman plot
Interpretation
On average Cuff_Device reads 2.03 units lower than Wrist_Device, a statistically significant bias. For 95% of cases the difference falls between -8.36 and 4.29. Whether that range is acceptable is a clinical judgment, not a statistical one. The differences change with the size of the measurement (proportional bias).
How to cite this tool
Using these results in a thesis, journal article, report or class assignment? Please cite this free tool so others can find it. It keeps the site free.
APA 7
Xavier, B. (2026). Bland-Altman Plot and Analysis [Web application]. FreeStatisticalTools.com. https://freestatisticaltools.com/bland-altman-plot/
BibTeX
@software{xavier_blandaltman_2026,
author = {Xavier, Bibin},
title = {Bland-Altman Plot and Analysis},
publisher = {FreeStatisticalTools.com},
year = {2026},
url = {https://freestatisticaltools.com/bland-altman-plot/},
note = {Web application}
}Suggested sentence for your methods section: “The analysis was carried out with the free Bland-Altman Plot and Analysis tool on FreeStatisticalTools.com (Xavier, 2026).”
What is Bland-Altman analysis?
Bland-Altman analysis assesses agreement between two methods that measure the same quantity. It plots the differences against the means and reports the bias and limits of agreement.
When to use Bland-Altman analysis
Use it to compare a new measurement method with an established one. A correlation coefficient is not a measure of agreement.
Assumptions
- Differences approximately normal.
- No strong trend of the differences with the mean (proportional bias).
How to use this tool
- Enter your data. Paste columns from Excel, Google Sheets or SPSS with variable names in the first row, or upload a CSV or Excel file. The example data shows the expected layout.
- Choose the variables. Select the method a and method b.
- Read the results. Results update automatically. Read the plain-English interpretation, then use Copy table to paste APA-style tables into Word and Copy for the results sentence.
Frequently asked questions
Why not use correlation to compare methods?
Two methods can correlate highly yet disagree by a constant or proportional amount. Bland-Altman analysis measures agreement directly.
What are limits of agreement?
The range bias ± 1.96 × SD of the differences, within which 95% of differences between the methods are expected to fall.
What is proportional bias?
When the difference between methods grows or shrinks with the size of the measurement, shown by a significant slope of differences on means.
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