An influential paper used this specification

GDPpc = Debt pc + Capital pc (Pc = Per Capita)

to estimate the impact of debt on GDP by way using the Common Correlated Effects (CCE) , the Mean Group (MG) and 2FE estimators with ECM model

1. Anyone with knowledge on how to get estimates using these estimators ?

2. The authors used per capita of the variables assuming a constant returns to scale. If i add other variables like trade, inflation or population growth to the above specification, will i be violating the constant returns to scale assumption ?