Visualizing interactions with twoway contour

[Marcus Eklund]

Hi!

I would love to get some help with how to visualize my interaction. I have been using this stata manual of twoway contour (https://www.stata.com/stata-news/news32-1/spotlight/). But the problem is that my graph does not show any curvature but only straight lines, making me wonder if it does not show the interaction (see picture). I use an education variable with six levels, and an income variable with 12. Can't see why my graph would differ. There is a significant interaction term. Happy for any advice!

reg outcomevar income education c.education#c.income
quietly margins, at(education=(1(1)6) income=(1(1)12)) saving(predictions, replace)

use predictions, clear
list _at1 _at2 _margin in 1/5

rename _at1 education
rename _at2 income
rename _margin pr_outcomevar

*reg outcome c.education##c.income
twoway contour pr_outcomevar income education, ccuts(0(0.2)5.0)),
xlabel(1(1)6)
ylabel(0(1)12, angle(horizontal))
xtitle("Education (categories)")
ytitle("Income")




Best,
Marcus

[Maarten Buis]

Hard to say without the data. Maybe your interaction term is statistically significant,but substantively zero. Maybe, the data is not what you think it is.