I have a very basic question regarding modelling when testing for interaction for finding heterogeneity in treatment effects in subgroups.
As far as I understand the general specification of the model where a single single subgroup is considered will be
Treat+Subgroup+Subgroup x Treat
However when analysing multiple subgroups what would be the correct way to specify the model
Treat+Subgroup1+Subgroup2+Subgroup1 x Treat
Treat+Subgroup1+Subgroup2+Subgroup1 x Treat+Subgroup2 x Treat