Re: st: count (Stata 8)

["Andres E. Hernando" <hernando@fas.harvard.edu>]

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...?
>
> Thanks for your help,
> Heather
>
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--------End of Message--------

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