Dear Prof Lachenbruch,
If I ignore the covariate and regress the one test against the other
they correlate pretty well.
I actually have the values of the covariate so - if I regress the
first test with it there is a significant slope; regressing the second
test against the covariate does not have a slope. I can not directly
compare the intercepts as the tests report in different units (this is
why I converted the results of both tests into corresponding
z-scores). An interaction of group/test and covariate is significant
but hard to visualize.
Do you think the approach of plotting the residual of the two tests'
z-scores against the different levels of the covariate and
demonstrating a significant slope is appropriate?
Nikos
On Thu, Apr 11, 2013 at 1:25 PM, Lachenbruch, Peter
<Peter.Lachenbruch@oregonstate.edu> wrote:
> sounds like there might be two approaches here:
> ignore the covariate and do a regression
> do the regression with the covariate fixed at the mean value in the first group (or even get the values)
> and compare the intercepts. Probably should use an interaction of group and coavariate
>
> Peter A. Lachenbruch,
> Professor (retired)
> ________________________________________
> From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] on behalf of Joseph Coveney [stajc2@gmail.com]
> Sent: Wednesday, April 10, 2013 7:40 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Re: z-score diff plots
>
> Nikos Kakouros wrote:
>
> I'd like to first apologize upfront for the lack of specificity in my
> question - please be sensitive to my lack of experience with Stata. :)
>
> I have two tests that are meant to measure the same thing (blood
> thickness) but with totally different units. On the whole they
> correlate well (patients with thick blood have higher values on both
> measurements), but the first test is affected (on univariate analysis)
> by another blood parameter but the second test is not. Their
> correlation is, therefore, dependent on the third parameter.
>
> I would appreciate on comments/advice on how to best visually show
> this relationship in Stata. I was thinking the following: convert all
> measurements to z-scores and depict the difference in z-score for the
> two measurements at different values (y axis) vs the value of the
> third variable that affects one of the tests more than the other
> (x-axis).
>
> Is anyone familiar with such a visualization?
>
> --------------------------------------------------------------------------------
>
> I'm not familiar with it. Would a scatterplot matrix (SPLOM) help?
> (See -help graph matrix-.)
>
> Joseph Coveney
>
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