What does a Bland-Altman plot show?

A Bland-Altman plot is a useful display of the relationship between two paired variables using the same scale. It allows you to perceive a phenomenon but does not test it, that is, does not give a probability of error on a decision about the variables as would a test.

How do you read a Bland and Altman plot?

If the points on the Bland–Altman plot are scattered all over the place, above and below zero, then it suggests that there is no consistent bias of one approach versus the other. It is, therefore, a good first step for two measurement techniques of a variable.

How do you make a bland-Altman graph?

  1. A Bland-Altman plot compares two assay methods.
  2. Create a new table.
  3. Enter the measurements from the first method into column A and for the other method into column B.
  4. Designate the columns with the data (usually A and B), and choose how to plot the data.
  5. The Bland-Altman analysis creates two pages of results.

What is proportional bias in Bland Altman?

The Bland-Altman analysis may bring proportional bias, which is present when the difference in values resulting from two methods increases or decreases in proportion to the average values.

What is a good limit of agreement?

The 95 percent ‘limits of agreement’ is calculated as the mean of the two values, minus and plus 1.96 standard deviations. This 95 percent limits of agreement should contain the difference between the two measuring systems for 95 percent of future measurement pairs.

How do you calculate Bland Altman limits of agreement?

The Bland–Altman method calculates the mean difference between two methods of measurement (the ‘bias’), and 95% limits of agreement as the mean difference (2 sd) [or more precisely (1.96 sd)]. It is expected that the 95% limits include 95% of differences between the two measurement methods.

How do you interpret passing bablok?

Passing & Bablok (1983) make a stronger statement “If we accept both b=1 and a=0 we can infer y*=x* or, in other words, both methods are identical”. If 0 is not in the CI of a there is a systematic difference and if 1 is not in the CI of b then there is a proportional difference between the two methods.

What does proportional bias mean?

Proportional bias means that one method gives values that are higher (or lower) than those from the other by an amount that is proportional to the level of the measured variable.