Lab-Tacular

Bland-Altman Plotter

Analyses the agreement and the regression between two measurement methods.Paste the values measured by the two methods as two columns.

The left column is X and the right column is Y. Differences are always Y − X, so putting the established method in X makes the sign read as “how much higher the new method runs”.

One sample per line, the two values separated by a tab, a space or a comma. Copying two columns straight out of a spreadsheet works. Calculated from 3 pairs and accepted up to 500.

How this is calculated — Bland-Altman and Passing-Bablok

A correlation coefficient does not answer whether two methods agree: two methods that are always 5 units apart correlate perfectly. So this tool reports Bland-Altman, which shows how far apart they are, alongside Passing-Bablok, which shows whether that distance is constant.

Bland-Altman takes the difference (Y − X) and the average of each pair, then reports the mean of the differences and the limits of agreement at mean ± 1.96 SD. The standard deviation is the sample one (n − 1).

Passing-Bablok is a non-parametric regression for the case where both axes carry error. It takes the slope between every pair of points and uses their median — a shifted median, because the pairwise slopes are not independent and the plain median would be biased. The intercept is the median of y − b·x.

  • Pairs with the same x and the same y — the same point twice — are dropped. Pairs with the same x but different y are kept as a vertical slope.
  • Pairs whose slope is exactly −1 are dropped, as the paper directs. That is what makes the fit of Y on X agree with the fit of X on Y.
  • The shift K is the number of pairwise slopes below −1. Some packages define that step differently, which can move the last digits, so N and K are shown on screen.
  • Confidence intervals follow the rank-order method from the original paper, two-sided 95%. No bootstrap is used.
  • When the difference visibly grows with the size of the measurement it is usual to re-read Bland-Altman on a percentage scale; this tool does not offer that view yet.

Frequently asked questions

References

Where the two methods and the reference values come from.

  1. Bland JM, Altman DG. (1986) Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 327(8476):307–310.doi:10.1016/S0140-6736(86)90837-8The bias and the "bias ± 1.96 SD" limits of agreement — 1.96 is part of the definition, not a rounding
  2. Passing H, Bablok W. (1983) A new biometrical procedure for testing the equality of measurements from two different analytical methods. Application of linear regression procedures for method comparison studies in clinical chemistry, Part I. J Clin Chem Clin Biochem 21(11):709–720.doi:10.1515/cclm.1983.21.11.709The shifted median of pairwise slopes, the rules that drop identical points and slopes of exactly −1, and the 95 % confidence interval from rank statistics
  3. Wicklin R. (2022-02-14) Passing-Bablok regression in SAS. The DO Loop (SAS blog).https://blogs.sas.com/content/iml/2022/02/14/passing-bablok-regression-sas.htmlThe reference values — the post's two datasets (102 and 50 pairs) and the slope, intercept and confidence intervals printed in its text, reproduced to the digits given

CLSI EP09 is mentioned only as the context for the 40-specimen recommendation; the document was not used as a comparison target.

Often used together

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