st: Events before D: how do I analyse?

[Tim <lists@timbp.com>]

I have a dataset formed by data linkage of hospitalisation data and 
defined by people requiring ongoing (chronic) treatment D.

It has been suggested that people in remote areas who require D move 
closer to services in the months before needing D (or in the next few 
months).
Also, group A form a greater proportion of remote populations and are 
more likely to require D and are more likely to move.

My dataset includes the area where the person was living at each 
observation (hospitalisation) time.

I want to know if people in more remote areas are more likely to move 
(closer to services) in the 6 or 12 months before requiring D. I also 
want to know if people in group A are more likely to move (closer to 
services) in the 6 or 12 months before requiring D.

The data are left censored; they only include people who require D, but 
some of those require D on or before their first record, so I have no 
idea when they first required D.

So I want to analyse events before the index (defining) event. 
Furthermore, I'm not actually interested in time; I want to know about 
incidence rates.

I can -stset- with -exit(`my_index_event')- with enter at first record,  
but what happens with those who move multiple times before D? I am only 
interested in time from most recent move to D. Can I -stset- to 
recognise only the latest event as the event?

It seems to me that, given everyone in my dataset has experienced D, I 
can reverse time to calculate incidence of moving in previous 6 or 12 
months. The situation looks completely symmetrical to me, but I cannot 
find any reference that I can use for support.



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