Graphing a logistic regression model with a quadratic term

[Chris Hearne]

I have a quadratic regression model that includes a quadratic term. I know that the command

twoway qfit yvar xvar

will plot a model with a quadratic term in it, but upon closer inspection this is for linear models, not logistic ones. What is the appropriate way of plotting this relationship? Appreciate your guidance in this.

[Clyde Schechter]

Like this:

sysuse auto, clear
logistic foreign c.mpg##c.mpg // AND OTHER TERMS IF YOU LIKE
margins, at(mpg = (15(5)40))
marginsplot

Note: for this to work you absolutely must use factor variable notation to include the quadratic. If you try -gen mpgsq = mpg*mpg- and -logistic foreign mpg mpgsq-, then -margins- and -marginsplot- will give you gibberish. If you are not yet familiar with factor variable notation do read -help fvvarlist- and the corresponding manual section: this is one of Stata's most useful features and every regular Stata user should learn it.

Also note that -marginsplot- accepts most of the options available with -graph twoway-, so you can customize the appearance of the graph to your taste.

[Chris Hearne]

Clyde, thanks so much, that was very clear.