Dear Sutirtha,
What follows is personal opinion.
The main purpose of a Bland-Altman difference plot is to allow the user
to inspect the data from a method comparison (or similar) study in a
more detailed manner than in a standard x-y scatter plot. The graph
rotates the line of identity round to the horizontal and then expands
the scatter around the line allowing easier viewing (using the space in
the graph in a better way). By contrast, with an x-y plot it can be hard
to see the nature of the scatter of results, especially when the scatter
round the line is small compared with the range of values.
So the first thing is visual inspection. Is the data scattered evenly
around the line across the range (indicating a linear relationship with
small proportional or absolute bias, or is there another pattern: eg
proportional bias (data shows a slope); absolute bias (data not near
zero at low end); non-linearity (eg above the line in the middle and
below at either end); are there outliers or uneven scatter suggesting
blunders or differences in analytical specificity. Applying "+/-2 SD"
lines to the data when there are these kinds of observations seems
clearly wrong to me (in spite of appearances in journals).
If the data does not suffer these problems then the line indicating the
midpoint of the differences describes the expected average difference
and the "+/- 2SD" lines illustrate the expected scatter. These then must
be compared with clinical requirements.
Regards,
Graham
>>> "Dr.Sutirtha Chakraborty" <[log in to unmask]>
18/06/2013 2:20 am >>>
Can anyone guide be how to interpret bland altman difference plots.
How do we know from this plot(bias and 2 SD limits) that the agreement
between 2 methods is suitable for clinical use.
Regards,
Sutirtha
Regards,
Dr Sutirtha Chakraborty
Sent on my BlackBerry® from Vodafone
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