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st: test for percentage change


From   "Jonathan A. Schwabish" <jschwabi@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: test for percentage change
Date   Wed, 27 Apr 2005 12:38:44 -0700 (PDT)

This may be more of a statistics question than a Stata
question but was hoping someone might be able to help.

I am trying to test the statistical significance of
the percent change in the number of observations
between two years and the median income between two
years. 

For the first, I assume I should use the hypothesis
test for a proportion:

Z = p1-p2 +/- 1.96*sqrt[p1*(1-p1)/n1 + p2*(1-p2)/n2],

where p1=#obs in year1/total #obs in year 1; and
p2=#obs in year2/total #obs in year 2. A value of Z
less than 1.96 would reject the null that the percent
change is statistically significant.

For the latter, I've used the -median- command to test
whether the two medians are the same. If the test
rejects the null that the two medians are the same,
does this imply that the percent change is
statistically significant? If not, what test should I
use?

Thanks,
jon

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