the conditions for stationarity may be difficult to grasp,but looking at some pictures may help.in figure 16.1 a-b we plot some artificially generated,stationary time series.note that the series vary randomly at a constant level (mean) and with constant dispersion(variance). in figure 16.1 c-d are plots of series that are not not stationary. these time series are called random waiks,because they slowly wander upwards or downwards,but with no real pattern. in figure 16.1 e-f are two more nonstationary series, but these show a definite trend either upwards or downwards. these are called random waiks with a drift.