Dear Stata Members
I have seen the following model based on a paper

Code:
Delta Cashit= b0+ b1EPUit+ b2Cash flow from operationsit+  b3EPUit * Cash flow from operationsit +Controls+Country fixed effects+Industry fixed effects+Year fixed effects
EPU denotes Economic policy uncertainty index developed by Baker et al 2016

The data comprises of some 50 countries and 200,000 firms in total for the period 2000-2020.
My doubts are as follows
1) Can we include both industry fixed effects and country fixed effects since industry fixed effects subsume the power of country fixed effects? Is there an exception for this?
2) There will case (mostly such cases will be prevalent) where each firm i in year t is assigned the same EPU value of year t. This means one cannot include year fixed effects (FEs) in the models where EPU is specified because the EPU variable is cross-sectionally invariant, hence year dummies will subsume all the explanatory power of PU(Duong et al 2020). So, can we add year dummies?
3) Clustering is done at the country level. I think this is correct since we allow errors to be correlated for observations belonging to the same countries but not observations belonging to different countries.
4) Is it not better to account for just firm dummies(no country or industry dummies or time dummies)?


Any help in this regard will be highly helpful