Hi!

What could be the idea behind interact GDP with Quarter Fixed Effects in a Difference in Difference model (GDPfor2014i *QuarterFEt)?
I understand that treatment could somehow effect GDP if it was not interacted but is there any more explanation to why it is a good idea to interact GDP with Quarter Fixed Effects?


Total passengers aviation = B0 + B1Swedeni+ B2After2018Q2it + B3(Swedeni*After2018Q2it) + B4QuarterFEt+ B5CountryFEi+ B6(GDPfor2014i *QuarterFEt)+uit

Total passengers aviation: Total passengers that travel with aviation
Treatmeant group = Sweden
Treatment = A tax was implented at 2018 in the secound quarter

Regards
David