Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
| 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 >>>> >>>> * >>>> * For searches and help try: >>>> * http://www.stata.com/help.cgi?search >>>> * http://www.stata.com/support/faqs/resources/statalist-faq/ >>>> * http://www.ats.ucla.edu/stat/stata/ >>> * >>> * For searches and help try: >>> * http://www.stata.com/help.cgi?search >>> * http://www.stata.com/support/faqs/resources/statalist-faq/ >>> * http://www.ats.ucla.edu/stat/stata/ >> * >> * For searches and help try: >> * http://www.stata.com/help.cgi?search >> * http://www.stata.com/support/faqs/resources/statalist-faq/ >> * http://www.ats.ucla.edu/stat/stata/ > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/faqs/resources/statalist-faq/ > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/