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


From   Vũ Võ <hoangvu@ueh.edu.vn>
To   statalist@hsphsun2.harvard.edu
Subject   st: How to store marginal effect value after using margin command?
Date   Sat, 8 Jun 2013 22:50:16 +0200

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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