st: RE: test difference of two linear combinations that are differences of means

["Schaffer, Mark E" <M.E.Schaffer@hw.ac.uk>]

David,

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Radwin, David
> Sent: 06 February 2014 21:03
> To: statalist@hsphsun2.harvard.edu
> Subject: st: test difference of two linear combinations that are differences of
> means
> 
> 
> Dear Statalisters,
> 
> How do I test whether the difference of two linear combinations is statistically
> significant if the linear combinations are differences of means?
> 
> In the contrived example below, I want to test whether the 0.08 absolute mean
> difference between the South and West in mean January temperatures is
> different than the 8.88 absolute mean difference between the South and West
> in July temperatures.
> 
> I can't use -suest- as some earlier posts have suggested because I get the error
> message, "impossible to retrieve e(b) and e(V) in A" (error 198).

-suest- works after -regress- (among other commands).  Can you rejig your example so that the means are estimated by -regress- and reported as coefficients?  E.g., instead of

mean tempjan, over(region)

use

reg tempjan ibn.region, nocons

HTH,
Mark

> 
> David
> --
> David Radwin, Senior Research Associate
> Education and Workforce Development
> RTI International
> 2150 Shattuck Ave. Suite 800, Berkeley, CA 94704
> Phone: 510-665-8274
> 
> www.rti.org/education
> 
> . sysuse citytemp, clear
> (City Temperature Data)
> 
> . mean tempjan, over(region)
> 
> Mean estimation                     Number of obs    =     954
> 
>            NE: region = NE
>     _subpop_2: region = N Cntrl
>         South: region = South
>          West: region = West
> 
> --------------------------------------------------------------
>         Over |       Mean   Std. Err.     [95% Conf. Interval]
> -------------+------------------------------------------------
> tempjan      |
>           NE |   27.88537   .2766693      27.34241    28.42832
>    _subpop_2 |   21.69437   .3397395      21.02764    22.36109
>        South |    46.1456   .6568972      44.85647    47.43473
>         West |   46.22539   .7033823      44.84503    47.60575
> --------------------------------------------------------------
> 
> . lincom [tempjan]West - [tempjan]South
> 
>  ( 1)  - [tempjan]South + [tempjan]West = 0
> 
> ------------------------------------------------------------------------
> ------
>         Mean |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
> Interval]
> -------------+----------------------------------------------------------
> ------
>          (1) |   .0797906   .9624243     0.08   0.934    -1.808925
> 1.968506
> ------------------------------------------------------------------------
> ------
> 
> . mean tempjuly, over(region)
> 
> Mean estimation                     Number of obs    =     954
> 
>            NE: region = NE
>     _subpop_2: region = N Cntrl
>         South: region = South
>          West: region = West
> 
> --------------------------------------------------------------
>         Over |       Mean   Std. Err.     [95% Conf. Interval]
> -------------+------------------------------------------------
> tempjuly     |
>           NE |      73.35   .1843789      72.98816    73.71184
>    _subpop_2 |   73.46725   .1841403      73.10589    73.82862
>        South |    80.9896   .1881789      80.62031    81.35889
>         West |   72.10859   .4051957      71.31342    72.90377
> --------------------------------------------------------------
> 
> . lincom [tempjuly]West - [tempjuly]South
> 
>  ( 1)  - [tempjuly]South + [tempjuly]West = 0
> 
> ------------------------------------------------------------------------
> ------
>         Mean |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
> Interval]
> -------------+----------------------------------------------------------
> ------
>          (1) |  -8.881006   .4467604   -19.88   0.000    -9.757754
> -8.004258
> ------------------------------------------------------------------------
> ------



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