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


From   David Hoaglin <dchoaglin@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Interpretation of the Bland-Altman method on Stata
Date   Wed, 21 Nov 2012 23:37:44 -0500

Paul,

Isn't it possible to read Emily's message differently?  She could be
saying that, when she transformed the difference in means into a ratio
(i.e., inverted the logarithm), she got 0.34, and then multiplying by
100 gave 34.  Then "the questionnaire is 34% of the record" could be
the right interpretation.  (I have not used -baplot-, so I don't know
whether the interpretation would be that one or "the record is 34% of
the questionnaire.")

It definitely would help to see the commands and the relevant output.

David Hoaglin

On Wed, Nov 21, 2012 at 11:24 AM, Seed, Paul <paul.seed@kcl.ac.uk> wrote:
> 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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