st: Assistance with xtmixed - categorical predictors, unequal observation intervals

["Benigno Rodriguez G., MD" <rodriguez.benigno@clevelandactu.org>]

I am reposting this with a modified subject line, as it did not show 
up on the list, and it originally contained "Help" as the first word 
of the subject, which is apparently problematic. I apologize if this 
ends up duplicated!

Hi,

I have a dataset that contains CD4 measurements on 400 unique persons 
at 10 unevenly spaced timepoints (2,4,16,40 weeks et cetera). I am 
interested in fitting a random intercept and slope model to these 
data, using the fixed effect "region" (categorical with 4 levels) and 
the random effect "BaselineCD4". The variable "week" contains the 
time, and the dataset is in the "tall" format. Could I please request 
help with the proper syntax?

The questions I want to address are:

1.- Does the estimated slope of CD4 vary by region (controlling for 
baselineCD4)? If so, which specific regions have an estimated larger 
of smaller slope (compared with region 1)
2.- Can I obtain an estimate of the magnitude of the effect of 
region, comparing each region to the reference (region=1)?
3.- Which covariance structure is best suited to the data? The 
correlation matrix is shown below.
4. The initial graphic assessment suggests that a quadratic model 
could fit the data better (an initial rapid increase in CD4, followed 
by a slower slope. How would this model be fitted?

. corr c4wk0 c4wk2 c4wk4 c4wk8 c4wk12 c4wk16 c4wk24 c4wk32 c4wk40 c4wk48
(obs=296)

             |    c4wk0    c4wk2    c4wk4    c4wk8   c4wk12   c4wk16 
  c4wk24   c4wk32   c4wk40   c4wk48
-------------+------------------------------------------------------------------------------------------
       c4wk0 |   1.0000
       c4wk2 |   0.8439   1.0000
       c4wk4 |   0.8400   0.8073   1.0000
       c4wk8 |   0.8112   0.7860   0.7757   1.0000
      c4wk12 |   0.8325   0.7969   0.8045   0.8269   1.0000
      c4wk16 |   0.8391   0.7715   0.7604   0.7960   0.8663   1.0000
      c4wk24 |   0.7731   0.7432   0.7551   0.7578   0.8095   0.7855   1.0000
      c4wk32 
|   0.7899   0.7499   0.7753   0.7581   0.8289   0.7894   0.8544   1.0000
      c4wk40 
|   0.7915   0.7430   0.7659   0.7717   0.8131   0.8075   0.7934 
0.8545   1.0000
      c4wk48 
|   0.7186   0.6755   0.6958   0.7451   0.7765   0.7487   0.7697 
0.8338   0.8612   1.0000

Any help would be tremendously appreciated!

Best,

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