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st: margins at() and margins over() give different graphs


From   Luca Fumarco <luca.fumarco@lnu.se>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: margins at() and margins over() give different graphs
Date   Sat, 1 Dec 2012 18:06:38 +0100

Dear Statalister,

In my analysis I make use of the probit model, and I am investigating the interaction between two variables, let's say X1 and X2, where X1 is a dummy variable while X2 is a continuous discrete variable.
I am using the "margins" command in order to see how X1 changes with changes in X2.

First, I type:
margins, dydx(X1) over(X2) vsquish
marginsplot

Then, since X2 does not have many values, I use the at() option instead of the over()
margins, dydx(X1) at(X2=(......)) vsquish
marginsplot

Since, in the second version, I have not forgotten any X2 values why do the 2 versions give a slightly different graph?
More precisely, the second graph gives a smooth curve, while the first one presents some kinks.

Thank you for your time

Luca Fumarco

Ph.D. Student
Linnæus University
School of Business and Economics

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