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Re: st: comparison between a repeated ordinal measures


From   Joseph Coveney <jcoveney@bigplanet.com>
To   Statalist <statalist@hsphsun2.harvard.edu>
Subject   Re: st: comparison between a repeated ordinal measures
Date   Wed, 30 Mar 2005 18:56:02 +0900

Joseph Wagner wrote

I need to do a comparison between two ordinal measures, one at baseline
(hlths) and the other, repeated at followup(f6hlths).  I have done
something similar in SAS using CATMOD.  I wish to know if there has been
a change between the two time points and in which direction.


The data takes this form:


Self Rated |    6M Self Rated Health
    Health |    1     2    3    4    5 | Total
-----------+--------------------------+-----
         1 |   28    18    6    0    0 |    52
         2 |   21    78   44    1    0 |   144
         3 |    7    34   96    5    1 |   143
         4 |    0     3   18   16    0 |    37
-----------+--------------------------+-----
     Total |   56   133  164   22    1 |   376


Is the command -mvrepeat- that Philip Ender wrote, appropriate?

----------------------------------------------------------------------------

In this case, -mvrepeat- would give the same answer as -ttest- using the
paired t-test syntax.  I vaguely recall reading that under these
circumstances Student's t-test does surprisingly well with ordinal data with
as few as three categories, but consider using an alternative, such as a
nonparametric test or a modeling command intended for ordered categorical
data.  There are several of each from which to choose.  In addition
to -ologit- (illustrated below), Stata has user-written commands that don't
rely upon the proportional odds assumption, at least one of which
(-gologit-) allows the -cluster()- option.

To observe the direction of change and its magnitude, you can either
use -predict- after one of the modeling commands or plot the data using a
graphing command specifically for ordered categorical data.  (I've
illustrated using -ordplot-, but be aware that its author, Nick Cox, has
enhanced it and updated it for Stata Release 8 under the name
of -distplot-.)

Joseph Coveney

clear
set more off
input byte sco0 byte cou1 byte cou2 byte cou3 byte cou4 byte cou5
1 28 18  6  0  0
2 21 78 44  1  0
3  7 34 96  5  1
4  0  3 18 16  0
end
reshape long cou, i(sco0) j(sco1)
drop if cou == 0
expand cou
drop cou
signtest sco0 = sco1
signrank sco0 = sco1
generate int pid = _n
reshape long sco, i(pid) j(tim)
somersd tim sco, cluster(pid)
ologit sco tim, cluster(pid)
npt_s sco, by(tim) strata(pid) nodetail
version 7: ordplot sco, by(tim)
gllamm sco tim, i(pid) family(binomial) link(ologit)
estimates store A
gllamm sco, i(pid) family(binomial) link(ologit)
estimates store B
lrtest A B, stats
exit


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