Dear research community,

what are the Assumptions of the unit root test for the NARDL Model?

I have reviewed the literature, and some studies assume stationary of variables at the first difference and apply the NARDL model. Whereas, some studies consider stationary of variables at the level, also at the first difference and apply the NARDL model.

So, for the NARDL model, the variables should be at the first difference OR should be mixture of level and First difference?