I have a database of applicants for a grant and their applications for each year. I want to know the effect of the grant on nr of publications. But first i want to perform a common trend analysis. i did this in STATA.
My dependent variable is nr_publ = number of publications.
My independent variable is grant = 0 for non-granted, 1 for granted
I created time dummies for each year in my pre-treatment period ( year of application - 5 until year of application + 2) and each year in my post-treatment period (year of application +2 until year of application +8. represented by tt_X = years before application, tt0 year of application and ttX year of application
Secondly, i created interaction terms of the time dummies and the receipt of grant (also dummy which is 1 when the researcher obtained a grant and 0 if he did not), by grant * timedummy, again grant_tt_x = x years before application, grant_tt0 = year of application and grant_ttX is x year after application
i thought the appropriate regression was:
xtreg nr_publ grant tt_4 tt_3 tt_2 tt_1 tt0 tt1 tt2 tt3 tt4 tt5 tt6 tt7 tt8 grant_tt_4 grant_tt_3 grant_tt_2 grant_tt_1 grant_tt0 grant_tt1 grant_tt2 grant_tt3 grant_tt4 grant_tt5 grant_tt6 grant_tt7 grant_tt8,robust fe
i left out the first pre-treatment year (tt_5, and the interaction term for this year grant_tt_5)
However, this gives me a negative estimates for all interaction terms which is unlikely. I assume that i did something wrong.
can somebody explain to me, what is the correct regression? or what i did wrong?