Re: st: margins after oprobit with interactions

[Richard Williams <richardwilliams.ndu@gmail.com>]

At 10:16 AM 11/21/2011, Vincenzo Carrieri wrote:
> Dear Statalisters,
> I am estimating an ordered probit model:
>
> Happy=age+age^2+income+income*age+income*age2 , that in "factor 
> model style" is:
> oprobit happy c.age c.age#c.age c.income  c.income#c.age c.income#c.age#c.age
>
> where Happy is a 5 point scale oredered variable. I would like to have
> marginal effects of all these variables. I am using the command
> margins. So for instance, for the top category I write:
>
>  margins , dydx(*) predict (outcome(5))
>
> Stata gives me only two marginal effects: one for age and  one for
> income. If I have understood well, these marginal effects are
> estimated by STATA taking into account the fact that income is
> interacted with age, age is squared and income is interacted also with
> age squared. Is it right?
>
> In addition, why I don't get also the marginal effect of the
> interaction terms? If I do the first derivatives of the model, I get
> dy/dincome, dy/dage but also dy/dagedincome. IS there a way to get
> marginal effect for interaction terms with margins, or any other
> command in Stata?
You can get marginal effects for interactions with the old -mfx- 
command. Unfortunately, they are wrong. You might want to read
http://www.stata-journal.com/article.html?article=st0063

Variations of this Q pop up regularly. To me, it makes no sense to 
want the marginal effect of an interaction term. The value of the 
interaction can't change without one of the terms used to compute the 
interaction changing. Any effect the interaction has is captured in 
the marginal effects for the terms used to compute the interactions. 
So, either there is no marginal effect for the interaction; or, if 
you do something like the above article does, you get a marginal 
effect for each case. But again, no single term. If I am wrong about 
this I'd love to have somebody point out the error in my thinking.

-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu
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