I have annual data with 25 observations. 1 dependent variable and 4 independent variables. I used to methods to test stationarity. First I de-trended data and used ADF test and found out that all my variables became stationarity at level I(0) after de-trending. I also directly applied ADF to all the variables but then some became stationary at first difference, some at level and some at second difference. Now, I am confused which stationarity criteria to take for further analysis. And depending on whichever approach I take, what model should then I apply to estimate the parameters?
P.S:- Also, I applied johanssen-cointegration test on my original data and found that variables are con-integrated of order 2. That is, 5-3=2.
Help needed urgently. thanks