st: Comparing continuation rates for drugs with different durations of use

[Paul O Brien <paul.obrien@afe2.org.uk>]

Database of visits to clinic for two contraceptivesa new implant  
which last for 3 years and the contraceptive injection which needs to  
be topped-up every 12 weeks.

I want to compare the continuation rates for new users starting year  
2000.

The start date for the implant is the date of insertion and terminal  
date the day of removal.

The difficulty is with the second method, the injection. The start  
date it the date of first injection (i.e. has not had an injection in  
the previous 16 weeks), and the terminal date is 12 weeks after the  
last injection, if she has not had another within 16 weeks. A patient  
who returns for injection 15 week after the last one is considered to  
be still using the method, but has discontinued if she return 16  
weeks after the previous one. How do I identify the start and end dates?

Those still using will be censored.

Thanks.

Paul

-------------------------------
Paul O Brien
Raymede Clinic
Exmoor St
London
W10 6DZ
44 + (0)20 + 8962 4450




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