st: Simple longitudinal data question – drug continuation rate

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

I have a database of clinic visits with 3 variables, patient_id,  
datevisit, and drug.

I want to know the number of patients who were prescribed drug  
‘microg’ in the first half of 2005 were also prescribed the same drug  
during a visit in the first half of 2006.

Or is there a better way to calculate the one-year continuation rate?

Patients can make any number of visits.

Thanks,

Paul

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