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st: RE: Confidence intervals differ between Predict, Adjust, and T-test, which to use?
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From |
"Martin Weiss" <[email protected]> |
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To |
<[email protected]> |
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Subject |
st: RE: Confidence intervals differ between Predict, Adjust, and T-test, which to use? |
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Date |
Tue, 20 Jan 2009 18:17:30 +0100 |
Line for the server...
You are not comparing " predict, adjust, and t-test command" but " ci,
adjust, and t-test command". Otherwise it would be weird that -adjust-,
which is a front end for -predict-, throws up different results. -ci- and
-ttest- assume normality while adjust after -qreg- employs the results
handed to it by -qreg-... I also think that -qreg- assumes the 50% quantile
as default which need not coincide with the mean...
HTH
Martin
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of I M
Sent: Tuesday, January 20, 2009 6:11 PM
To: [email protected]
Subject: st: Confidence intervals differ between Predict, Adjust, and
T-test, which to use?
Hello,
After I run a quantile regression estimate, I am unsure of how to summarize
the results. By using the predict, adjust, and t-test command, I arrive at
the same mean, but different 95% CI. Can anyone tell me why the 95% CI are
different, and which one to use if I want to summarize the predicted mean?
---begin---
sysuse auto, clear
qreg price weight length foreign
adjust, by(foreign) ci
predict adjustedprice
by foreign, sort: ci adjustedprice
ttest adjustedprice, by(foreign)
---end---
adjust command:
Domestic | 5301.89 [4803.46 - 5800.31]
Foreign | 5879.43 [5108.19 - 6650.67]
predict command:
Domestic | 5301.886 [4745.151 - 5858.621]
Foreign | 5879.43 [5342.07 - 6416.79]
t-test command:
Domestic | 5301.886 [4745.151 - 5858.621]
Foreign | 5879.43 [5342.07 - 6416.79]
Note how the calculated mean is the same for each, but the 95% CI is
different in each case. Why are they different, and which one do I use if I
want to summarize the predicted mean?
Thanks!
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