Re: st: difference-in-difference

[Chris Rohlfs <car@uchicago.edu>]

Hans,

This is generally done using a regression.  I'm not quite sure what the 
format of your data is from this output, but let's say that you reshape 
the data into "long" format, so that you have two dummy variables:

post - equals 1 if the observation is in the post period & 0 otherwise.
treatment - equals 1 if the observation is in the treatment group and zero 
otherwise.

Then do:

	gen pt = post*treatment
	reg gruppe pt post treatment

The coefficient on pt is the difference-in-difference estimator.  The 
t-statistic on that regression coefficient is the t-test for equality of 
the differences.

Chris

On Fri, 24 Jun 2005, Hans J. Baumgartner wrote:


> Dear statalist,
>
> I am trying to obtain the difference-in-difference estimator by using
> ttest. However, I got stocked somewhere half way.
>
> I ttested the difference in two groups. (see below). What I would like to
> do now is to check the difference of these two differences. Is there
> anyway to do this?`
>
> . ttest gruppe11 = gruppe10, unpaired
>
> Two-sample t test with equal variances
>
> ------------------------------------------------------------------------------
> Variable |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf.
> Interval]
> ---------+--------------------------------------------------------------------
> gruppe11 |      36    .4444444    .0839921    .5039526    .2739314
> .6149575
> gruppe10 |     203    .4187192    .0347119    .4945689     .350275
> .4871634
> ---------+--------------------------------------------------------------------
> combined |     239    .4225941    .0320194    .4950087    .3595164
> .4856719
> ---------+--------------------------------------------------------------------
>    diff |            .0257252    .0896915               -.1509692
> .2024197
> ------------------------------------------------------------------------------
> Degrees of freedom: 237
>
>               Ho: mean(gruppe11) - mean(gruppe10) = diff = 0
>
>     Ha: diff < 0               Ha: diff != 0              Ha: diff > 0
>       t =   0.2868                t =   0.2868              t =   0.2868
>   P < t =   0.6127          P > |t| =   0.7745          P > t =   0.3873
>
> . ttest gruppe01 = gruppe00, unpaired
>
> Two-sample t test with equal variances
>
> ------------------------------------------------------------------------------
> Variable |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf.
> Interval]
> ---------+--------------------------------------------------------------------
> gruppe01 |      57    .7017544    .0611344    .4615545    .5792875
> .8242213
> gruppe00 |     285    .5017544    .0296694    .5008764    .4433546
> .5601541
> ---------+--------------------------------------------------------------------
> combined |     342    .5350877    .0270098    .4994981     .481961
> .5882144
> ---------+--------------------------------------------------------------------
>    diff |                  .2    .0717663                .0588382
> .3411618
> ------------------------------------------------------------------------------
> Degrees of freedom: 340
>
>               Ho: mean(gruppe01) - mean(gruppe00) = diff = 0
>
>     Ha: diff < 0               Ha: diff != 0              Ha: diff > 0
>       t =   2.7868                t =   2.7868              t =   2.7868
>   P < t =   0.9972          P > |t| =   0.0056          P > t =   0.0028
>
>
>
>
> ++++++++++++++++++++++++++++++++
> Hans J. Baumgartner
> Dept. Public Economics
> DIW Berlin
> German Institut for Economic Research
> phone: +49/30/89789-307
> fax :      +49/30/89789-114
> URL: www.diw.de
>          www.hansbaumgartner.de
> ++++++++++++++++++++++++++++++++
> *
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