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

From   "Seed, Paul" <>
To   Emily Watson <>
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 
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 

Best wishes, 

Paul Seed

From: Emily Watson [] 
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

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