RE: st: nonfactorial combinations with -margins-

["Feiveson, Alan H. (JSC-SK311)" <alan.h.feiveson@nasa.gov>]

Rich - That's it! Didn't know the "at" could be repeated.

Thanks.


Al

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Richard Williams
Sent: Thursday, August 01, 2013 3:02 PM
To: statalist@hsphsun2.harvard.edu; statalist@hsphsun2.harvard.edu
Subject: Re: st: nonfactorial combinations with -margins-

What is with the numbers on at1 and at2? Is this what you want?

margins, at(headroom = 2.4 weight = 2000 length = 150 ) at(headroom =
3.6 weight = 3000 length = 200 )

At 01:29 PM 8/1/2013, Feiveson, Alan H. (JSC-SK311) wrote:
>Hi - In Stata 13, I've been struggling with getting -margins- to 
>produce arbitrary combinations of several predictors without creating a 
>huge list of all possible combinations.
>
>
>Example:
>
>. sysuse auto,clear
>(1978 Automobile Data)
>
>. reg price headroom weight length
>
>       Source |       SS       df       MS              Number of 
> obs =      74
>-------------+------------------------------           F(  3,    70) =   13.23
>        Model |   229789847     3  76596615.8           Prob > 
> F      =  0.0000
>     Residual
> |   405275549    70   5789650.7           R-squared     =  0.3618
>-------------+------------------------------           Adj R-squared =  0.3345
>        Total |   635065396    73  8699525.97           Root 
> MSE      =  2406.2
>
>------------------------------------------------------------------------------
>        price |      Coef.   Std. Err.      t    P>|t|     [95% 
> Conf. Interval]
>-------------+---------------------------------------------------------
>-------------+-------
>     headroom
> |  -486.4429   388.7626    -1.25   0.215    -1261.806    288.9197
>       weight
> |   4.674047   1.118074     4.18   0.000     2.444119    6.903975
>       length
> |  -87.59202   39.88962    -2.20   0.031    -167.1494   -8.034676
>        _cons
> |   9969.584   4304.018     2.32   0.023      1385.49    18553.68
>-----------------------------------------------------------------------
>-------
>
>
>
>
>Suppose I want to get predictions at only two combinations of these 
>three predictors:(headroom = 2.4, weight = 2000, length = 150) and 
>(headroom = 3.6, weight = 3000, length = 200). But if I specify
>
>
>. margins,at(headroom = (2.4 3.6) weight = (2000 3000) length = (150 
>200) )
>
>Adjusted predictions                              Number of obs   =         74
>Model VCE    : OLS
>
>Expression   : Linear prediction, predict()
>
>1._at        : headroom        =         2.4
>                weight          =        2000
>                length          =         150
>
>2._at        : headroom        =         2.4
>                weight          =        2000
>                length          =         200
>
>3._at        : headroom        =         2.4
>                weight          =        3000
>                length          =         150
>
>4._at        : headroom        =         2.4
>                weight          =        3000
>                length          =         200
>
>5._at        : headroom        =         3.6
>                weight          =        2000
>                length          =         150
>
>6._at        : headroom        =         3.6
>                weight          =        2000
>                length          =         200
>
>7._at        : headroom        =         3.6
>                weight          =        3000
>                length          =         150
>
>8._at        : headroom        =         3.6
>                weight          =        3000
>                length          =         200
>
>------------------------------------------------------------------------------
>              |            Delta-method
>              |     Margin   Std. Err.      t    P>|t|     [95% 
> Conf. Interval]
>-------------+---------------------------------------------------------
>-------------+-------
>          _at |
>           1  |   5011.412   616.6852     8.13   0.000     3781.473 
>    6241.352
>           2  |   631.8115    1654.47     0.38   0.704    -2667.925 
>    3931.548
>           3  |    9685.46   1488.435     6.51   0.000      6716.87 
>    12654.05
>           4  |   5305.859   655.2722     8.10   0.000     3998.959 
>    6612.758
>           5  |   4427.681   810.5569     5.46   0.000     2811.076 
>    6044.286
>           6  |    48.0801   1620.927     0.03   0.976    -3184.757 
>    3280.917
>           7  |   9101.728   1584.549     5.74   0.000     5941.445 
>    12262.01
>           8  |   4722.127   581.5246     8.12   0.000     3562.313 
>    5881.941
>-----------------------------------------------------------------------
>-------
>
>
>
>I get all 8 combinations of these three variables at two values each, 
>yet I only care about two of them. Clearly this can get out of hand if 
>there are lots of predictors or values in these lists.
>
>
>What I would like is something like
>
>. margins,at1(headroom = 2.4 weight = 2000 length = 150  ) at2(headroom 
>= 3.6 weight = 3000 length = 200  )
>
>
>but this is not allowed.
>
>Is there a syntax for doing what I want that I have overlooked?
>
>Thanks
>
>
>Al Feiveson
>
>
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-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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