Hi Susan,
An equivalent method would be to use linear regression with dummy coding for the treatment groups. Ignoring the issue of bootstrapping, in Stata this would be:
sysuse auto, clear
xi: regress weight i.foreign
where -xi- expands terms containing categorical variables into indicator variables, and -i.foreign- is the indicator of treatment assignment. If you type
ttest weight, by(foreign)
you can see the estimate of the mean difference is the same as the beta for _Iforeign_1 from the regression.
To bootstrap the standard errors type
xi: bootstrap, seed(12345) reps(100): regress weight i.foreign
which will give the confidence intervals you desire. Setting the seed ensures that you can reproduce the results later. The -reps- command sets the number of repetitions.
-- Matt
[email protected]
-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Susan Maree Cotton
Sent: Monday, 18 August 2008 10:48 AM
To: [email protected]
Subject: st: Bootstrapping confidence interval of mean difference
Importance: High
Hi All,
I have data from a randomized clinical trial with intervention and treatment as usual groups. The outcome data is cost-effectiveness data. Reviewers of a manuscript pertaining to data from this trial have requested that the bootstrapped confidence interval of the mean difference between groups be reported.
I am a new user to STATA and have been having difficulties determining the correct commands to generate these confidence intervals. I have worked out how to bootstrapping for the confidence intervals of the means for the two groups separately and to bootstrap the t-distribution pertaining to difference between groups but cannot work out the confidence interval for mean difference. Please help!!
Thanks in advance.
KR
Sue
Dr Sue Cotton
Senior Research Fellow
ORYGEN Research Centre
Department of Psychiatry
University of Melbourne
Locked Bag 10 (35 Poplar Road)
Parkville Victoria
Australia 3052
Phone: +61 3 9342-2859
Fax: +61 3 9342-2941
Mobile: 0407-340-115
[email protected]
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