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st: Is this a situation for mlogit? xt-flavor of mlogit?


From   "Ploutz-Snyder, Robert (JSC-SK)[USRA]" <robert.ploutz-snyder-1@nasa.gov>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Is this a situation for mlogit? xt-flavor of mlogit?
Date   Mon, 2 Aug 2010 14:40:08 -0500

Stata Colleagues;

I'm at a loss for how to analyze the following data and am looking for suggestions.  Everything that I think of seems to have flaws... maybe I'm over thinking things a bit?  Maybe  I just need more coffee?


Experimental Design: 3(times)x2(groups)x4(celltypes) mixed factorial. (random ID: repeated on time & celltypes)

More specifically
   ->Blood data are collected from children at roughly three times per child (varname = age).  
          (Experimental protocol was to collect data at approx 12, 18 & 24 months of age, 
           but we have true age recorded in the dataset. There are some missing data.)

   ->Two groups of kids (independent measures factor. Varname = group)

   ->At each of the three repeated times, my outcomes data are the proportion of cells 
     belonging to each of 4 categories.  Thus the 4 "categories" of blood cells total to 1 (within 
     rounding error) per child, per time point.  (varname=y)


We want to examine/compare the effects of group, age  and group#age on the distribution among the 4 cell type.


I don't typically work with this kind of data, but I think it is a situation in which mlogit would be appropriate, Yes?

If so, is there an xt flavor of mlogit that would be better than mlogit with robust errors?

Rob






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