st: plotting interaction in Tobit analysis

 From frone@ria.buffalo.edu To statalist@hsphsun2.harvard.edu Subject st: plotting interaction in Tobit analysis Date Wed, 4 Jun 2003 15:34:06 -0400

```I'm aware of how to plot an interaction from linear regression, but I'm
having some trouble figuring out how to plot an interaction from a Tobit
regression where the predicted values are constrained to fall within the
range of the observed outcome.

For example, my variables are:

alc=alcohol use
race=race (0=white, 1=minority)
pa=postive affect (scale scores range from 1 - 4)
int=race*pa

My Tobit results are:

alc = 1.37 + .42(pa) - 5.89(race) + 1.84(int)

One can rewrite this equation to get the separate equations for white and
minority:

white: alc = 1.37 + .42(pa)

minority: alc = -4.52 + 2.26(pa)

Now consider the equation for minorities, if I compute the predicted value
for alc at the lowest level of pa, I get

minority: alc = -4.52 + 2.26(1) = -2.26

Of course, the lower limit of the observed variable is zero unlike the
latent outcome.

Is there a way to plot this interaction where the predicted alcohol values
fall within the range of the observed outcome?

Thanks in advance for any help.

Mike Frone

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
```