Re: st: Quick question of using -concord- to investigate proportional errors

[Nick Cox <njcoxstata@gmail.com>]

You have been posting to the list since October. See my reply to you
last Saturday spelling out the longstanding and often repeated request
to explain where user-written programs come from.

http://www.stata.com/statalist/archive/2012-01/msg00517.html

In this case -concord- is a program last updated in SJ 10(4).
-concord- is a small toolkit for thinking quantitatively about the
structure of agreement or disagreement between two continuous
measures. Homing in on significance test results misses most of what
-concord- can tell you.

You only cite part of your results but they show only weak agreement
between your measures: the concordance correlation will be about .262.
The low P-value of 0.002 just says that the concordance correlation is
definitely not zero; that is a conventional test for those who need
it, but is unlikely to be an answer to the underlying scientific
question.

Your results also show a correlation between difference and mean that
is a bit high.

So there is a lot of scatter in your data and the error structure is
only roughly multiplicative; there's some extra structure on top of
that.

Most of what is written on -concord- is freely available in the STB
and SJ archives, so I won't repeat explanations already given.

Nick

On Thu, Jan 19, 2012 at 12:29 AM, cecilia sam <samcecilia2010@gmail.com> wrote:

> I am new to statistics and Stata, and need help for using -concord- in
> Stata 11.0 for Windows.
>
> I am comparing two datasets derived from 2 different methods. I
> natural log transformed both datasets on the original scale, and typed
> -concord y x, loa(regline)-. The output provides the slope of the
> regression line, and a p-value listed besides the Pearson r (I
> attached a part of the Stata output and attached below). Is this the
> p-value that indicate the signficance of the slope of the regression
> line?  In this case, does the p-value <0.05 mean that there are
> statitically signficant proportional errors ?
>
>
> Pearson's r =  0.271  Pr(r = 0) = 0.002  C_b = rho_c/r =  0.966
>
> Reduced major axis:   Slope =     1.301   Intercept =    -2.727
>
> Difference = y - x
>
>         Difference                 95% Limits Of Agreement
>
>    Average     Std Dev.             (Bland & Altman, 1986)
>
> ---------------------------------------------------------------
>
>      0.010       0.333                 -0.643      0.663
>
> Correlation between difference and mean = 0.266
>

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