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st: count (Stata 8)


From   Heather Gold <heg2001@med.cornell.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: count (Stata 8)
Date   Tue, 10 May 2005 10:54:07 -0400


Dear Listers -

I know this must be simple, but something eludes me.
I have a data set of subjects (cases and non-cases) by year and am trying to calculate a prevalence per year, ie, [cases/(cases+noncases)].
ie, data look like this:
id year dxflag
1 1995 1
1 1996 0
1 1997 0
2 1995 0
2 1996 1
...

I found that I can write
by year: count if dxflag==1
or
by year: count

and these return number of cases in the first line, and count of total population in the second line, just as I want.
But what I need are these values as scalars, so I can create a prevalence per year. In the Stata 8 help/manual it says that -count- saves the scalar of number of observations in r(N), but I can't seem to get this number to appear. Do I actually have to write a program to calculate this? And how does the "by year" part fit in...?

Thanks for your help,
Heather

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