Interact a dummy against its own continuous

[Pratap Pundir]

I find that it is very hard to interpret quadratic parameters.

When I can see an inflection point in the scatter plot, does it make any sense at all to create a dummy variable such that higher than inflection x's are 1, and lower are 0 and then to interact that with the continuous?

e.g. say i have income in $ and age in days
maybe i can see in scatter plot that the income age chart inflects at around 50 years of age (18250 days).

gen age_high = 0
replace age_high = 1 if age > 18250

Does it make any sense, and is it even possible to run a model:

reg income age##age_high

[Maarten Buis]

I suspect that what you are looking for is a linear spline. See help mkspline

[Pratap Pundir]

Yes, thanks! I think I AM basically looking for a piecewise technique.