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st: Choice of analysis time in survival analysis where the hazard is associated with age

From   roland andersson <>
Subject   st: Choice of analysis time in survival analysis where the hazard is associated with age
Date   Mon, 14 Dec 2009 13:38:56 +0100

I have a large national dataset and plan to analyse socioeconomic
differences in risk of having appendicitis. Persons born after 1949 to
end of 1990 and alive at 1/1/1991 (n=4.500.000) are included in the
dataset. The dataset contains the date when a person was operated for
appendicitis between 1/11/1990 and 31/12/2003 and date of death.

The hazard of having appendicitis is agedependent and is increasing
till age 13 and then decreasing. Previous collaborators have made
Coxregression with time from 1/11/1990 to the appendicitisdiagnosis or
censoring as analysistime, and have entered age at operation as
covariates to control for the age-dependent differencies in hazard.

Knowing that the hazard is age-dependent I think this may give biased
results. I therefore wonder if it would be better to use age as
analysis time (with late entries) instead of time from start of follow
up with adjustment for age. Am I right to think that this will be a
better way of controling for the age-dependent differences in hazard?
If that is the case may I also include age as a covariate in the
analysis or would that complicate things?

I have also considered to use a stratified analysis with ageintervals
as stratification variable. Can you give some advice on how these
ageintervals should be chosen in view of the agedependent increasing
and decreasing hazard?

I would appreciate your comments.

Roland Andersson, MD PhD
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