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st: different age effect between groups


From   Enzo Coviello <enzo.coviello@alice.it>
To   statalist@hsphsun2.harvard.edu
Subject   st: different age effect between groups
Date   Mon, 30 Oct 2006 07:27:45 +0100

Dear Stata Users,

I have a Cox model where women invited to test screening (mammography) are compared for cause specific mortality with women not invited. The age ranges from 50 to 69.

I checked the functional form of the variable age using gam.
The age effect is linear in not invited cases.

In invited the age effect rises until 55 and then flattens or even decreases as shown in the strate output:

. strate inv agegroup,per(1000)

failure _d: status2004
analysis time _t: (exit-origin)/365.25
origin: time datainc
id: ID

Estimated rates (per 1000) and lower/upper bounds of 95% confidence intervals
(9251 records included in the analysis)

+---------------------------------------------------------------------+
| inv agegroup D Y Rate Lower Upper |
|---------------------------------------------------------------------|
| 0. Not Invited 50- 132 5.5111 23.952 20.195 28.407 |
| 0. Not Invited 55- 131 4.7708 27.458 23.137 32.587 |
| 0. Not Invited 60- 182 5.8115 31.317 27.083 36.214 |
| 0. Not Invited 65- 234 6.5993 35.458 31.194 40.305 |
|---------------------------------------------------------------------|
| 1. All Invited 50- 90 5.9692 15.077 12.263 18.537 |
| 1. All Invited 55- 153 7.5791 20.187 17.229 23.653 |
| 1. All Invited 60- 142 7.6450 18.574 15.757 21.895 |
| 1. All Invited 65- 124 8.0587 15.387 12.904 18.348 |
+---------------------------------------------------------------------+


This effect in invited women to a test screening is described in a paper I read.


My question is how to model this different age effect in invited and not invited women.
Probably I need some interaction term.
Have you some advice about how to generate it/them?

Thanks in advance for your attention.

Enzo

^^^^^^^^^^^^^^^^
Enzo Coviello
Dipartimento Prevenzione ASL BAT/1
Ufficio Igiene Pubblica
via Barbera 27 - 70055 MINERVINO MURGE (BA)
Italy
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