On 20 Mar 2018, at 05:58, VINCENT KOPPELMANS <vincent.koppelmans at utah.edu> wrote:
Dear all,
I am looking for advice on how to run a repeated analysis on count data.
My issue is as follows:
* We counted the number of carotid plaques from ultrasound images in a diseased population and a group of control subjects
* We have measured the plaques for all subjects in both the left and the right carotid artery during the same session
* The number of plaques is a count score ranging from 0 to 6
* The distributions look like this:
* Plaques in the left carotid artery: https://www.dropbox.com/s/t5tqh4wjrfc5eml/Left.png?dl=0
* Plaques in the right carotid artery: https://www.dropbox.com/s/nl5ezef145av2ae/Right.png?dl=0
* (Where NKI= the diseased population; RSS= the control subjects; (all)= the two groups combined. There are no numbers on the x-axis, but the 7 columns are the count scores 0-6 (left to right).)
* There is biological evidence that the distribution of plaques for left and right differ in the general population (i.e., our control subjects).
* I would like to test if the difference in distribution of plaque scores between the left and right carotid arteries is different between my two populations.
* Previous analyses (e.g., comparing a single side between groups) showed me that a negative binomial distribution is a better fit for my data than a Poisson distribution.
My idea is to run a repeated-measures negative binomial regression analysis where plaque score measures (left and right) would be the repeated measures. In this case I would be interested in the ?body side? by group interaction.
My questions are:
* Is a good and valid approach?
* I am thinking about using R?s GEE package (https://cran.r-project.org/web/packages/gee/index.html). Would that be the right tool for this job?
Thanks!
- Vincent
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