Hello.

I have a question with respect to using the lincom command after the margins command. Specifically:
Code:
margins, dydx(x1 x2) at(x1=(-1 1) m=1) at(x2=(-1 1) m=1)
The output is the following:
Code:
Expression   : Linear prediction, predict()
dy/dx w.r.t. : x1 x2

1._at        : x1              =          -3
               m               =           1

2._at        : x1              =           3
               m               =           1

3._at        : m               =           1
               x2              =          -3

4._at        : m               =           1
               x2              =           3

------------------------------------------------------------------------------
             |            Delta-method
             |      dy/dx   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
x1           |
         _at |
          1  |   .0162047    .015535     1.04   0.297    -.0142706      .04668
          2  |  -.0174807   .0155769    -1.12   0.262    -.0480383    .0130768
          3  |   -.000638   .0056649    -0.11   0.910     -.011751     .010475
          4  |   -.000638   .0056649    -0.11   0.910     -.011751     .010475
-------------+----------------------------------------------------------------
x2           |
         _at |
          1  |   .0018624   .0049187     0.38   0.705    -.0077867    .0115115
          2  |   .0018624   .0049187     0.38   0.705    -.0077867    .0115115
          3  |  -.0216832     .01337    -1.62   0.105    -.0479114     .004545
          4  |    .025408   .0133222     1.91   0.057    -.0007265    .0515425
------------------------------------------------------------------------------
I want to compare now 1._at in x1 (.0162047) with 3._at in x2 (-.0216832), and 2._at in x1 with 4._at in x2. I use the two commands below:
Code:
lincom [x1]_b[1._at]-[x2]_b[3._at]
lincom [x1]_b[2._at]-[x2]_b[4._at]
but get a message:
Code:
equation [x1] not found
r(303);
Would be grateful for any help.