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
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...?