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st: RE: Interpretation of the Bland-Altman method on Stata


From   "Seed, Paul" <paul.seed@kcl.ac.uk>
To   Emily Watson <emilywatson632@gmail.com>
Subject   st: RE: Interpretation of the Bland-Altman method on Stata
Date   Wed, 21 Nov 2012 16:24:22 +0000

Dear Emily, 

(I received this privately, but am posting my answer to Statalist
to be shared.)

You might post such questions on Statalist 
See http://www.stata.com/support/faqs/resources/statalist-faq/
for information on how to joins and how to write posts.

In any case, please the read the FAQ in detail, in particular 
the advice about how to ask a question.

A difference of 34 on the logged scale is massive. Astronomical.
Larger than the US national debt.
It corresponds to a ratio of 10^34 (if you are working 
with logs base 10) or exp(34) ), which is roughly 5.8*10^14 
(if you are using natural logs). 
Either number is so large that I am sure you have an error 
somewhere else.

My command -baplot- is not designed for handling logged values.
And I can't say what the answer means without seeing the question.

The general advice to show exactly what you typed and what happened 
applies.

Best wishes, 

Paul Seed


From: Emily Watson [mailto:emilywatson632@gmail.com] 
Sent: 21 November 2012 03:16
To: Seed, Paul
Subject: Interpretation of the Bland-Altman method on Stata

Hi Paul,

I came across your email address when searching for help on Stata - I hope you don't mind me emailing you.

I am using baplot to compare two methods of measuring dietary intake. I have entered the code -baplot record questionnaire- (record and questionnaire being my two methods(or variables)), both of which are the log versions.

I am of the understanding when log versions are used, once the output is backtransformed the output will be a ratio which when multiplied by 100 is then converted to a percentage.

If the mean difference is then 34 (after the above adjustments) does this mean the questionnaire is 34% higher than the record? or the record is 34% higher than the questionnaire? or the questionnaire is 34% of the record?

Thank-you for your time and help

Kind regards
Emily


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