# Re: st: count (Stata 8)

 From "Andres E. Hernando" To statalist@hsphsun2.harvard.edu Subject Re: st: count (Stata 8) Date Tue, 10 May 2005 11:24:15 -0400

Hi,

This would do it... but only for the last year:

. sort year
. by year: count if dxflag==1
. scalar cases = r(N)
. by year: count
. scalar total = r(N)
. scalar prev = cases/total
. di prev

basically r(N) stores only the last scalar reported so, eventually you'd need to put the above code within a forvalues/foreach cycle, maybe storing results in a vector.

But why to do it in this way? what's wrong with using a couple of egen and gen operators? I'm thinking:

. sort year
. by year: egen cac = count(year) if dxflag==1
. by year: egen cas = max(ct)
. drop cac
. by year: egen tot = count(year)
. gen prev = cas/tot
. by year: su prev

Best,
Andres.

Quoting Heather Gold, who wrote:
-------Original Message-------

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...?

Heather

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