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Re: st: one-way 95 CI for Ha: diff < 0


From   Svend Juul <SJ@SOCI.AU.DK>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: one-way 95 CI for Ha: diff < 0
Date   Sat, 14 Jun 2008 23:07:38 +0200

 
Carlo wrote:
 
after performing the following ttest, I would like to know about a possible
way to obtain with Stata 9.2/SE the one-way 95 CI for Ha: diff < 0.
 
ttest qaly, by(treat) unequal
 
Two-sample t test with unequal variances
----------------------------------------------------------------------------
Group      Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
0      250       1.1472   .0158906    .2512527     1.115903
1.178497
1          250       1.1872   .0157876     .249624     1.156106    1.218294
----------------------------------------------------------------------------
Combined  500     -.0399999    .0224                  -.0840101    .0040102
----------------------------------------------------------------------------
diff = mean(0) - mean(1)                                      t =  -1.7857
Ho: diff = 0                     Satterthwaite's degrees of freedom =497.979
Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
Pr(T < t) = 0.0374         Pr(|T| > |t|) = 0.0748          Pr(T > t) =0.9626
 
===========================================================
 
I am not sure that a "one-way CI" has any reasonable interpretation. But
you might get what you aim at by a 90% confidence interval. Rothman and 
Greenland prefer 90% CIs - at least they did; I don't have their 3rd 
edition here (see http://www.stata.com/bookstore/me.html) - probably 
from similar reasons as yours.
 
   ttest qaly, by(treat) unequal level(90)
 
Hope this helps
Svend
 
________________________________________________________ 
 
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6 
DK-8000 Aarhus C,  Denmark 
Phone, work:   +45 8942 6090 
Phone, home:   +45 8693 7796 
Fax:           +45 8613 1580 
E-mail:        sj@soci.au.dk 
_________________________________________________________ 

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