Dear _all,
This might be a silly question, but I currently have no access to
Stata's user manuals or stats textbook.
I have data on two independant samples (of different sizes), each
observed over 20 periods. I wish to draw a graph representing, for each
period, the difference in the proportion of some event between the two
groups, together with the 95% CI for this difference.
I first wanted to use -prtest- for each period, store/compute the
differences and CI bonds in new variables, and graph them against periods.
However, I am a bit confused over which standard error to use (and how
to get them) when computing the CI bounds.
-prtest- gives two standard errors, the one under H0 can be easily
computed from stored results r(P_1), r(P_2) and r(z). The standard
errors for each proportion (se1 and se2) are not stored, and I do not
know if the fact that my calculation of sqrt(se1^2 + se2^2) differs from
the reported standard error for the difference (not under H0) is caused
by rounding errors or if my formula is incorrect...
I thought of using -proportion- to get the standard errors as stored
results, but it actually gives standard errors for each proportion that
differ from -prtest-, but are identical to the ones computed by -mean-.
Any advice would be appreciated.
Antoine
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