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Re: st: nonfactorial combinations with -margins-


From   Richard Williams <[email protected]>
To   [email protected], "[email protected]" <[email protected]>
Subject   Re: st: nonfactorial combinations with -margins-
Date   Thu, 01 Aug 2013 15:01:53 -0500

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
OFFICE: (574)631-6668, (574)631-6463
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