# RE: st: Interaction terms in a logit model

 From "Daniel Schneider" To Subject RE: st: Interaction terms in a logit model Date Sat, 19 Mar 2005 09:36:40 -0800

```Thanks Maarten,

But, is that really different from doing

When drep==0
prgen price, x(drep=0 drepXprice=0) f(3290) t(16000)

And a loop for all values of price using
prvalue, x(drep=1 price=\$value drepXprice=\$value)
When drep==1

And then using both results to generate the graph (sorry, my stata
programming skills are pretty non-existent, the previous lines is meant
as a a general statement not Stata code)?

Or, more specifically: is this enough to take into consideration the
issues raised by Norton, Wang, Ai?

Daniel

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of maartenbuis
> Sent: Saturday, March 19, 2005 1:40 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Interaction terms in a logit model
>
>
> --- "Daniel Schneider" <daniel.schneider@s...> wrote:
> > My problem is that I would like to just do a -predict- command,
> > generate some simple graphs that show the different behavior of the
> > predicted probability depending on the manipulation of the two
> > variables (e.g., if X2 is a binary dummy, I would get two lines
> > running from left to right, when the yaxis is my predicted
> probability
> > and the xaxis is every value that X1 can take).
>
> One way to do just that is to do it manually like in the code
> below. The predicted probability whithin a logistic regression is
> exp(xb)/(1+exp(xb)) In the example xb is _b[_cons]+_b[price]
> if drep==0 and _b[_cons]+_b[price]+_b[drep]+_b[drepXprice]*x
> if drep==1.
>
> clear
> sysuse auto
> gen drep = rep78>=4
> gen drepXprice = drep*price
> logit foreign price drep drepXprice
> sum price
> twoway function y = exp(_b[_cons]+_b[price]*x)/ /*
>       */ (1+exp(_b[_cons]+_b[price]*x)), range(3290 16000) || /*
>    */ function y = exp(_b[_cons]+_b[price]*x + _b[drep] +  /*
>       */ _b[drepXprice]*x) / /*
>       */ (1+exp(_b[_cons]+_b[price]*x + _b[drep] +  /*
>       */_b[drepXprice]*x)), range(3290 16000) /*
>       */ ytitle("Pr(foreign)") xtitle("price in \$") /*
>       */legend(label(1 "drep=0") label(2 "drep=1"))
>
> Hope this helps,
> Maarten
>
>
>
>
>
>
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```