R help - Jun 2010 - Recoding session date into session id in a longitudinal dataset

Hi,

I'm fairly new to R but I have a large dataset (300000 obs) containing
patient material. Some patients came 2-9 times during the three year
observation period. The patients are identified by a unique idnr, the
sessions can be distinguished using the session date. How can I recode the
date of the session to a session id (1-9). This would be necessary to obtain
information and do some analysis on the first occurence of a specific
patient or to look for trends.

Thanks

JP Bogers
University of Antwerp

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