| The logic for requiring stationarity is that models inferred from stationary series are also stationary or stable. |
| Given the nature of time-series data, it is necessary to test the stationarity of each individual series. |
| A stationarity requirement, defined as a constant mean, variance and autocorrelation through time, constrains parameters to a certain range. |
| Each variable also displays 1 behavior since first-differencing of level data restores stationarity, as shown above. |
| To ascertain nonstationarity or stationarity of time series variables, DF-GLS, Ng-Perron and KPSS tests are implemented. |
| Coefficients of the ARMA models are also checked for both stationarity and invertibility. |
