dear all,
thank you for your suggestions:
david - you were right on the mark. i have sorted the data according to date, time of surgery. luckily it is not more complicated than diathermy[2] and diathermy[2]. for the others (i.e. monopolar, bipolar occurences) thankfully there are no monopolar2 or bipolar2.
ashim, roy - your suggestions, which were similar, are what i was looking for - i have gone through the data, everytime the the first diathermy[2] occurred, it has been replaced by diathermy[1] leaving the second occurence as diathermy[2] when i used your suggestions.
nick - i think my possible rather emphatic intonation of "every first occurences" statement had misled you, hence the your referral to "How can I identify first and last occurrences systematically in panel data?" on http://www.stata.com/support/faqs/data/firstoccur.html, which was not the solution that i wanted.
regards,
bw
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