st: Linking a patient file with an event file

[Health Services PhD <hlthsrvcsphd@gmail.com>]

I want to link/merge several files, but my data are complicated and
I’m unsure how to proceed. I'm using Stata 11.2 for Windows.

My patient file contains sociodemographic information for ~60,000
patients. My event file contains information about ~20,000 visits to
emergency rooms. There is a patient ID variable in each file. Many
patients in my patient file will not be in my event file (because they
didn't visit the emergency room). My event file has a patient ID,
service date, and various other variables related to the visit.

I need to merge the files so I can analyze incidence and run
regression models. I've read lots about merging files, but haven't
seen anything that addresses my issue yet.

I assume I will need to reshape my event file from long to wide so
that there is one row per patient, but everything I read about
reshaping assumes that the multiple observations per patient are
identified somehow. So one thing I *think* I need to do is generate a
new variable that counts ED visits for each patient. I thought I might
be able to do something like this: (from
http://www.stata.com/support/faqs/data-management/creating-group-identifiers/)

by id2013: gen edcount = 1 if _n==1
replace edcount = sum(edcount)

But that gives me a sequential number that doesn't restart at 1 for
each new patient ID.

Is reshaping the right approach, and if so, how do I best create the
variable I need?

Thanks in advance for your time,
Lisa

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