Bland-Altman limits of agreement
Computes Bland-Altman limits of agreement between two measurement methods.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Mean of the differences, in the units measured
- -0.4
- Standard deviation of the differences
- 1.2
- Number of paired measurements
- 85
Result: limits of agreement spanning about 4.7, each with its own interval
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Bias
- The mean of the differences. Its sign says which method reads higher, and a bias can be corrected for.
- The limits
- Bias plus and minus 1.96 standard deviations of the differences. 95% of future differences are expected between them.
- Correlation
- Not agreement. Two methods can correlate almost perfectly and disagree by a clinically enormous margin.
- Acceptability
- A clinical decision, and one the papers say should be set before the study rather than read off the result.
- Uncertainty
- Each limit is itself an estimate, with a standard error of about the standard deviation times the square root of 3 over the number of pairs.
- A caution
- If the difference varies with the size of the measurement, one pair of limits is the wrong summary and only the plot shows it.
What you enter
- Mean of the differences, in the units measured
- Standard deviation of the differences
- Number of paired measurements
How this is calculated
Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1(8476):307-310. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8(2):135-160. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.