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# Re: st: How to store marginal effect value after using margin command?

 From William Buchanan To "statalist@hsphsun2.harvard.edu" Subject Re: st: How to store marginal effect value after using margin command? Date Sat, 8 Jun 2013 17:20:52 -0700

```If you're using the - margins - command, the - post - option is clearly documented in the help file.  Additionally, the Statalist FAQ suggests looking through the documentation as a good first step prior to querying the list.  If this isn't the answer that you were looking for it may be a good idea to be more explicit about your end goal.

HTH,
Billy

Sent from my iPhone

On Jun 8, 2013, at 16:54, Vũ Võ <hoangvu@ueh.edu.vn> wrote:

> Dear Sergiy,
>
> Thank you for your message. Probably, you misunderstand my question
> nor I am clear in raising a question.
>
> For example:
>
> sysuse auto, clear
>
> reg price price wgt mpg c.wgt#c.wgt c.mpg#c.mpg c.wgt#c.mpg
>
>
>      Source |       SS       df       MS              Number of obs =      74
> -------------+------------------------------           F(  5,    68) =   12.84
>       Model |   308384833     5  61676966.6           Prob > F      =  0.0000
>    Residual |   326680563    68  4804125.93           R-squared     =  0.4856
> -------------+------------------------------           Adj R-squared =  0.4478
>       Total |   635065396    73  8699525.97           Root MSE      =  2191.8
>
> ------------------------------------------------------------------------------
>       price |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
>      weight |  -31.88985   9.148215    -3.49   0.001    -50.14483   -13.63487
>         mpg |  -3549.495   1126.464    -3.15   0.002    -5797.318   -1301.672
>             |
>    c.weight#|
>    c.weight |   .0034574   .0008629     4.01   0.000     .0017355    .0051792
>             |
> c.mpg#c.mpg |   38.74472   12.62339     3.07   0.003     13.55514    63.93431
>             |
>    c.weight#|
>       c.mpg |   .5421927   .1971854     2.75   0.008     .1487154    .9356701
>             |
>       _cons |   92690.55   25520.53     3.63   0.001     41765.12      143616
> ------------------------------------------------------------------------------
>
> So, I would like to calculate the marginal effects of weight and mpg
> on price by using command:
> margins, dydx(weight mpg)
>
> and get ->
>
> Average marginal effects                          Number of obs   =         74
> Model VCE    : OLS
>
> Expression   : Linear prediction, predict()
> dy/dx w.r.t. : weight mpg
>
> ------------------------------------------------------------------------------
>             |            Delta-method
>             |      dy/dx   Std. Err.      z    P>|z|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
>      weight |   .5360966   .6516553     0.82   0.411    -.7411243    1.813318
>         mpg |  -262.0502    103.681    -2.53   0.011    -465.2612   -58.83919
> ------------------------------------------------------------------------------
>
>
> My question is how to obtain or store in to scalar or whatever in
> stata values of 0.5360966 and -262.0502 in the above table?
>
> If i use mfx , after that using matrix list e(b) as you suggested, it
> will give me coefficients of the regression. To some extent, it can
> say that these are marginal effets, but not what I would like to
> obtain.
>
> Any suggestion are highly appreciated. Thanks
>
> Vu
>
>> type either 'return list' or 'ereturn list' (depending on the command)
>> to see which results are saved after these commands. From what you
>> describe you are interested in matrices r(b) and e(b)
>>
>> sysuse auto, clear
>> generate lp=log(price)
>> regress lp weight length headroom mpg
>> mfx
>> matrix list e(b)
>>
>> Best, Sergiy
>>
>> On Sat, Jun 8, 2013 at 4:50 PM, Vũ Võ <hoangvu@ueh.edu.vn> wrote:
>>> Hi everyone,
>>>
>>> I am writing a code in do file to calculate the marginal effect of x
>>> on y (log-linear model)
>>>
>>> The function form as  following:
>>>
>>> Ln(Y) = a0 + a1*X + a2*X^2  + e ----> Y = exp(a0 + a1*X + a2*X^2)
>>>
>>> So, dY/dX = (a1 + 2*a2*X)*exp(a0 + a1*X + a2*X^2)        eq(1)
>>>
>>> If I run the regression by command line as:
>>>
>>> reg lnY x c.x#c.x
>>>
>>> after that using margins command:
>>>
>>> margins, dydx(x).
>>>
>>> I will obtain the marginal effect of x on Ln(Y):  (a1 + 2*a2*X).
>>>
>>> How can I calculate the dY/dX as in eq(1) by using the do file, not
>>> calculating manually?
>>>
>>>
>>> 2. Second question is:
>>>
>>> If I use command margins, predict(p)  and obtain the results as following:
>>>
>>> Marginal effects after hetprob
>>>       y  = Pr(y) (predict, p)
>>>          =  .54284206
>>> ------------------------------------------------------------------------------
>>> variable |      dy/dx    Std. Err.     z    p>|z|  [    95% C.I.   ]      X
>>> ---------+--------------------------------------------------------------------
>>>        x |   .3704576      .03237   11.44   0.000   .307015    .4339   .020114
>>>     xhet |  -.0736092      .02423   -3.04   0.002  -.121095 -.026124   .502716
>>>     off1 |  (offset1)                                                  1.00516
>>>     off2 |  (offset2)                                                  1.09709
>>> ------------------------------------------------------------------------------
>>>
>>> The question is how to store the marginal effects value, say in this
>>> case is 0.3704576,
>>>
>>> in the scalar in stata?
>>>
>>>
>>> Thank you so much.
>>>
>>> Vo Duc Hoang Vu
>>> *
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>>
>> *
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>
>
>
> --
>
> Vo Duc Hoang Vu
> Faculty of Development Economics
> University of Economics HCMC
> 1 A Hoang Dieu Street
> Phu Nhuan Dist. Ho Chi Minh
> Vietnam
> Tel: +84 94 550 22 77 (VN); +31 624 63 29 45 (NL)
> Fax: +84 8 38477948 (VN)
> E-mail:hoangvu@ueh.edu.vn
> http://www.voduchoangvu.net
>
> *
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*
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```