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st: longitudinal data


From   "Clyde Schechter" <clyde.schechter@einstein.yu.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: longitudinal data
Date   Mon, 13 Jun 2011 07:57:40 -0700

Much depends on details that you don't state in your post.  One thing that
can be said without further knowledge: your analysis will almost certainly
require you put your data in long form.  There just aren't a whole lot of
Stata analysis commands that work best with wide data.  See -help reshape-
for that.

I can't tell from your description what your outcome variable is.  Is it
the actual count of gametocytes, or is it the presence of any gametocytes
(i.e. count > 0)?  As you speak of prevalence in your post, I'm guessing
it's the latter.  If so, take a look at -xtlogit-.  If the former, have a
look at -xtpoisson- or -xtnbreg- for starters.

In any case, you also need to think about how to represent time in your
analysis.  What kind of trajectory of gametocyte counts (or histories of
presence/absence) need to be modeled: (logit-)linear in time, V-shaped,
upside-down V-shaped, U-shaped, more complicated, even arbitrary?  You
will need to look into splines perhaps (-help mkspline-), or for an
arbitrary time course represent time as a factor variable.

One more thing: my suggestions presume each subject is enrolled in only
one study arm.  If you have a crossover design, then there are further
complications and you may need to look at -xtmixed- or -xtmelogit- or
something like that.

Hope this helps get you started.

Clyde Schechter
Albert Einstein College of Medicine
Bronx, NY, USA



Original Message:

Hi all,
 I would like to get a hint in the easiest way to analyse this data, am
not very
good in STATA and more so with Biostatistics.
 I have a data set with data from an anti-malarial study with 3 arms
placebo(SP),
and two active arms, (SP+AS3) and (SP+AS2).
 gametocytes count is measured at days 0,1,2,7,14,21 and 28
The dataset is presented in a wide format
am intrested in effectiveness of treatment on gametocytes count and its
prevalence
during the 28 days
 1. How do i calculate odds ratio and proportions and ??2. p values of (SP
vs AS1)
and (SP vs AS2) and
 3. Corresponding p values of difference in the prevalence in those two
groups with
their respective 95% confidence intervals for the days above.

 Do I need to change the data set to a long format or I can do all this
analysis
with the current wide format.

Thanks

 Liwindi



Clyde Schechter, MA MD
Associate Professor of Family & Social Medicine

Please note new e-mail address: clyde.schechter@einstein.yu.edu

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