st: use of ttest to test difference of mean between the whole sample and a sub-sample.

["Alejandro Lopez-Feldman" <alelopez@ucdavis.edu>]

I have a question that is more about statistics than about Stata, but I hope
some of you can help me.  I want to test if the mean of a certain variable
is different at the regional level than at the national level.

Say for example that I am using the data set described in the Stata manual
under ttest example 4 (p.483)

use http://www.stata-press.com/data/r9/census

and I am interested in testing if the national mean of the variable medage
is different from the mean of the same variable but only considering region
1.  I can see how I could do this using the command ttest. My question is:
is it valid to use ttest in such a case?

Thanks,
Alejandro


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