Hello,

Im running a DID model( over two waves of a longitudinal survey) where the code that I use is;

Code:
reg  `y' i.treatment##i.post  $controls i.PERSONID , cl(PSUID)
Suppose I wanted to find the effect for various age cohort separately, which is generated using the following code,

Code:
recode agecohort(15/22=1) (23/29=2) (30/36=3) (37/43=4) (43/50=5) , gen(cbin)
the data is like this,

[CODE]----------------------- copy starting from the next line -----------------------
Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(agecohort cbin)
45 5
33 3
26 2
19 1
36 3
40 4
27 2
40 4
26 2
36 3
32 3
28 2
49 5
29 2
28 2
34 3
Should I run the code like this?
Code:
reg  `y' i.treatment##i.post  $controls i.PERSONID if cbin==1 , cl
or this
Code:
reg  `y' i.treatment##i.post##i.cbin  $controls i.PERSONID , cl
But if I do the latter, the coefficient in stata will not show for the cbin==1.

Please clarify what the difference is between the two.

Or third option is I define my treatment in a way that it is in cbin==1 ,2,3 respectively and do
Code:
reg  `y' i.treatment##i.post  $controls i.PERSONID , cl(PSUID)
separately for the differently defined treatments.

which one is the right way to go about it?