st: different age effect between groups

[Enzo Coviello <enzo.coviello@alice.it>]

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
tel. fax +39 0883 691053
tel (home) +39 0883 695055
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