Create a first.treat for DID with multiple periods

[Marcela Vieira]

Hi everyone,

I have a panel data (1960 to 1980) with multiple locations (lgacodes) and am trying to create a column with the first year of treatment for each location (to do Difference-in-Differences with multiple periods). I used this code:

bysort lgacodes: replace first_treat=min( year1_lga) if dummy_general==1

but it didn't work, so I used:

bysort lgacodes: replace first_treat= year1_lga if dummy_general==1

instead.

Now, I believe I have to create a foreach loop to replace 'first_treat' with the min (year1_lga) for each lgacodes, but not sure how to do it. Could you please help?

Many thanks,

Here is a sample of my data

[Øyvind Snilsberg]

Identify first year of treatment,

bys lgacodes (dummy): egen first_treat = min(year) if dummy

Replace missing values with nonmissing,

bys lgacodes (first_treat): replace first_treat = first_treat[1]