Re: st: Comparing results from two separate t-tests

[Geoff Wallace <gw46@cornell.edu>]

I took the advice of both Maarten and David to try a regression route 
and this definitely provides a lot more flexibility and indeed gives the 
same substantive results.

I pretty much followed Maarten's suggestion of incorporating the 
interaction effect and that does the trick for testing the hypothesis of 
any difference between liberals and conservatives on the treatment 
without have to do multiple tests.

Thanks again to both of you!

Cheers, Geoff

Maarten buis wrote:

> Assuming that you have two dumies called treatment (1=treatment,
> 0=control) and liberal (1=liberal, 0=conservative), and that your
> outcome variable is called y then you want:
>
> anova y treatment liberal treatment*liberal
>
> or:
>
> xi: regress y i.treatment*i.liberal
>
> The two are actually the same.
>
> If you want to read more about this, than the term you are looking for
> is "interaction effect".
>
> Hope this helps,
> Maarten
>
>
>
> --- Geoff Wallace <gw46@cornell.edu> wrote:
>
> > Dear All,
> >
> > I have had problems trying to test for whether the treatment effects
> > for 
> > two groups of respondents (conservatives and liberals) in a social 
> > science experimental questionnaire I am using are significantly 
> > different. I used the -ttest- command to obtain the difference of
> > means 
> > across the treatment and control group first for the conservative 
> > respondent subsample and then repeated the procedure for the liberal 
> > respondent subsample. I should also note that I used the –unpaired- 
> > option for each command since the data are not paired. The output
> > from 
> > the two -ttest- commands yields a separate treatment effect for each 
> > subsample. However, I am having trouble figuring out how to use Stata
> > to 
> > perform a corresponding ttest on the two separate difference of
> > means. I 
> > tried using -est store- after each -ttest- command, but as I expected
> >
> > this option does not appear to work with –ttest-.
> >
> > Any help would be much appreciated.
> >
> > Best, Geoff
> >
> > --
> > Geoffrey P.R. Wallace
> > Ph.D. Candidate
> > Department of Government
> > Cornell University
> >
> > 214 White Hall
> > Ithaca, NY 14850
> > t.202.302.4209
> > f.607.255.4530
> >
> > *
> > *   For searches and help try:
> > *   http://www.stata.com/support/faqs/res/findit.html
> > *   http://www.stata.com/support/statalist/faq
> > *   http://www.ats.ucla.edu/stat/stata/
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology
> Vrije Universiteit Amsterdam
> Boelelaan 1081
> 1081 HV Amsterdam
> The Netherlands
>
> visiting address:
> Buitenveldertselaan 3 (Metropolitan), room Z434
>
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
>
>       ___________________________________________________________ 
> Rise to the challenge for Sport Relief with Yahoo! For Good  
> http://uk.promotions.yahoo.com/forgood/
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
--
Geoffrey P.R. Wallace
Ph.D. Candidate
Department of Government
Cornell University

214 White Hall
Ithaca, NY 14850
t.202.302.4209
f.607.255.4530
www.people.cornell.edu/pages/gw46/
<http://www.people.cornell.edu/pages/gw46/index.htm>
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/