# Re: st: Question on Ttest.

 From Richard Williams To statalist@hsphsun2.harvard.edu Subject Re: st: Question on Ttest. Date Fri, 21 Nov 2003 08:17:54 -0500

Also, doing it the way you are doing it, I believe you could just multiply r(se) * square root of (N-1). e.g. in this case, Stata would be doing the calculation .2387615526929942 * sqrt(127) = 2.690706. (I know the logic of what to do, somebody who knows Stata better can probably figure out the exact commands!) RW

At 12:47 PM 11/21/2003 +0000, you wrote:

```Dear Statausers,
I'm doing a simple ttest, however I would like to retrive the Std. Deviation
of the difference of the means (see output below). Does anyone know if Stata
saves this statistic?
Many thanks,
JP

. ttest  match2291_0== match2291_1 if week>=2144

Paired t test

----------------------------------------------------------------------------
--
Variable |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf.
Interval]
---------+------------------------------------------------------------------
--
matc~1_0 |     127    76.95678    .9025185    10.17087    75.17072
78.74284
matc~1_1 |     127    59.05176    .9798913    11.04281    57.11258
60.99093
---------+------------------------------------------------------------------
--
diff |     127    17.90502    .2387616    2.690706    17.43252
18.37753
----------------------------------------------------------------------------
--

Ho: mean(match2291_0 - match2291_1) = mean(diff) = 0

Ha: mean(diff) < 0         Ha: mean(diff) != 0        Ha: mean(diff) > 0
t =  74.9912                t =  74.9912              t =  74.9912
P < t =   1.0000          P > |t| =   0.0000          P > t =   0.0000
r; t=0.18 12:41:07

. return list

scalars:
r(sd_2) =  11.0428143208466
r(sd_1) =  10.17086703109571
r(se) =  .2387615526929942
r(p_u) =  1.0480080349e-106
r(p_l) =  1
r(p) =  2.0960160698e-106
r(t) =  74.99123715134762
r(df_t) =  126
r(mu_2) =  59.05175589013287
r(N_2) =  127
r(mu_1) =  76.95678011075718
r(N_1) =  127

----------------------------------------

Joao Pedro W. de Azevedo
School of Business (Economics)
Ridley Building
University of Newcastle
NE1 7RU
United Kingdom

Telephone:  +44 (0) 191 222-5237
Mobile: +44 (0) 7984-060675
Fax:        +44 (0) 191 222-6548
E-mail: j.p.azevedo@ncl.ac.uk
http://www.students.ncl.ac.uk/j.p.azevedo/

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```
```-------------------------------------------
Richard Williams, Associate Professor
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```