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Re: st: Re: z-score diff plots


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Re: z-score diff plots
Date   Thu, 11 Apr 2013 16:18:24 +0100

I am not a statistician either. I'd encourage you to use your clincal
experience to evaluate what makes sense.

-pairplot- (SSC) appears to include this kind of plot.
Nick
njcoxstata@gmail.com


On 11 April 2013 16:13, Nikos Kakouros <nkakouros@gmail.com> wrote:
> Dear Prof Cox,
>
> Thank you for your reply.
>
> Although the tests are meant to measure the same factor they do so
> differently and hence with vary different resultant values.  This is
> why I standardized into z-scores and then plotted the difference
> between the matched z-scores against the third variable.
>
> I think it worked as I can see a pattern as expected from the below:
> a) The first test appears to be dependent on the third variable
> linearly (lower results, I think artifactual, with higher levels of
> the third variable).
>
> b) The second test values appear flat when plotted against the third
> varilable  (i.e the results from this test do not appear to be
> affected by this variable).
>
> c) The difference in z-scores between the two tests does have a
> significant slope (p<0.005) when plotted against the third variable.
> I take this to suggest that there is a significant difference in the
> way the two tests interact with the variable (else their z-scores
> should cancel out and produce a horizontal plot along the 0 line).  I
> guess this is effectively showing that there is a significant
> difference in their respective regression line coefficients against
> the third variable but in a graphical way.
>
> Does this make sense?  I appreciate this is not directly a Stata
> question but I'm a clinician with limited statistics experience and
> have never come across a similar plot and neither have I found it
> mentioned anywhere. I think that (if valid) it should be helpful...
> (maybe this could be the start of a new Stata command :)
>
> I would be grateful for your comments.
>
> Many thanks again,
>
> Nikos
>
>
>
> On Thu, Apr 11, 2013 at 10:55 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>> Plotting difference against mean is a standard plot for many people.
>>
>> It is a relative of  the principle that it is helpful to plot residual
>> against fitted.
>>
>> Plotting some difference against a third variable might similarly be
>> useful. It is a bit like the standard practice of plotting residuals
>> from some model against a predictor not in the model.
>>
>> The question is does it work, that is, can you see a pattern?
>>
>> There are some general references in
>>
>> SJ-4-3  gr0005  . . . . .  Speaking Stata: Graphing agreement and disagreement
>>         . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>>         Q3/04   SJ 4(3):329--349                                 (no commands)
>>         how to select the right graph to portray comparison or
>>         assessment of agreement or disagreement between data
>>         measured on identical scales
>>
>> .pdf accessible via http://www.stata-journal.com/article.html?article=gr0005
>>
>> Nick
>> njcoxstata@gmail.com
>>
>>
>> On 11 April 2013 15:24, Nikos Kakouros <nkakouros@gmail.com> wrote:
>>> Thanks, Joseph.
>>> I was hoping to have a single graph demonstrating the interaction
>>> between the three variables.
>>> I wonder what anyone thought of the proposed idea of plotting the
>>> z-score difference between the two tests against the interacting
>>> variable...
>>> KR
>>> Nikos
>>>
>>> On Wed, Apr 10, 2013 at 10:40 PM, Joseph Coveney <stajc2@gmail.com> wrote:
>>>> 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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