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Re: st: analysis of continuous gestational age


From   "Svend Juul" <[email protected]>
To   <[email protected]>
Subject   Re: st: analysis of continuous gestational age
Date   Sun, 1 Apr 2007 21:59:18 +0200

Ali wrote (after a suggestion to use -stcox- to study determinants of
gestational length (duration of pregnancies)):
 
That is true, specially with the fact that the exposure is during pregnancy so
it depends on gestational length. However, to avoid this I am looking at
exposures during the first 23 weeks' gestation were all women have the same
opportunity to  be exposed. when I want to see the effect of exposure on
prematurity (binary variable I used binomial regression instead of cox
regression by restricting the exposure to 23 weeks). In addition I want to see
the effect of exposure on gestational age (continuous). This calls for linear
regression-as I understand- but as I mentioned the residuals distribution is
not normal. In these cases a non-parametric analysis is required. I used qreg
but the results do not look right.

Since the dataset is very large does it matter if the residuals normal
distribution assumption was violated in linear regression?
--------------------------------------------------------------------
 
There is no doubt that the residuals after a linear regression are far from
a normal distribution. Also, there is hardly any meaningful transformation of
gestational age that could make this happen. So, relying on linear regression
is a problem.
 
-stci- can be used for simpler descriptions and comparisons, using e.g. the
median and other percentiles in a sort of non-parametric approach.
 
Good luck,
Svend

________________________________________________________ 
 
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6 
DK-8000 Aarhus C,  Denmark 
Phone, work:  +45 8942 6090 
Phone, home:  +45 8693 7796 
Fax:          +45 8613 1580 
E-mail:       [email protected] 
_________________________________________________________ 

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