st: Using nonparametric control function as regressor in -reg-, for RDD/DiD

[Jen Zhen <jenzhen99@gmail.com>]

Dear list members,

I would like to estimate the effect of a treatment, to which everyone
aged above 50 is assigned, while controlling sufficiently for the
effect of age per se.

Complication: My main sample is small, so if I estimate the effect of
A50 plus control functions for both sides using only the main sample I
lack statistical significance.
I do however have a placebo sample, which should plausibly have the
same effect of age per se, but in which being aged above 50 does not
lead to treatment assignment.

The first thing I did therefore was a DiD specification:
-reg outcome A50 Mainsample A50_Mainsample-, and I think that is doing
a good job.

However, since I observe age to the day, I think it might be useful in
addition to also try a specification with a continuous control
function in age. Given my sample size complications, I would like to
estimate this control function using also the observations from my
larger placebo sample, but estimate the effect of A50 only for the
main sample. I think I cannot do this by running -rd- on the pooled
sample, because there I cannot specify that I want the effect of A50
to be estimated for the main sample only.
So I have "manually" run
-reg outcome A50_Mainsample age age2 age3 A50*age A50_age2 A50_age3-

Now I would like to repeat this but replace the polynomials in age
with nonparametric control functions (within the reg command).
Is there any way to do this (unless you think this is the wrong thing
to do here anyway)?

Thank you so much and best regards,
JZ
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/