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# st: heteroskedasticity between groups, interpretation of -sdtest-

 From "Laura R." To statalist@hsphsun2.harvard.edu Subject st: heteroskedasticity between groups, interpretation of -sdtest- Date Mon, 17 Dec 2012 12:01:07 +0100

```Dear Stata users,

I have estimated a regression in which the cases stem from two surveys
that have been conducted in different years. I put a dummy variable
for the survey year into the regression

Now my questions:
1. Do I still have to worry about group heteroskedasticity, i.e. that
the estimations are somehow biased?

2. Does it matter if variances between these two groups are
homo/heterogenous for the descritive statistics? (I mean, when I
describe the mean of the dependent variable in these two years, and
make comparisons)

3. I did a test for heterogeneity between the variances between the
groups (-sdtest-), but how can I interpret the results?

E.g., I did: sdtest depvar, by(survey_1)

For example, I received this result:
Ha: ratio < 1               Ha: ratio != 1                 Ha: ratio > 1
Pr(F < f) = 0.8246         2*Pr(F > f) = 0.3508           Pr(F > f) = 0.1754

For some different data, this result:
Ha: ratio < 1               Ha: ratio != 1                 Ha: ratio > 1
Pr(F < f) = 1.0000         2*Pr(F > f) = 0.0000           Pr(F > f) = 0.0000

Thank you very much for your help.

Laura
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