Q & A and Summary Stationary and Wide Sense Stationary Process A stochastic process {…, X t-1 , X t , X t+1 , X t+2 , …} consisting of random variables indexed by time index t is a time series. The stochastic behavior of {X t } is determined by specifying the probability density or mass functions (pdf’s): p(x t1 , x t2 , x t3 , …, x tm ) for all finite collections of time indexes {(t 1 , t 2 , …, t m ), m < ∞} i.e., all finite-dimensional distributions of {X t }. A time series {X t } is strictly stationary if p(t 1 + Ï„, t 2 + Ï„, …, t m + Ï„) = p(t 1 , t 2 , …, t m ) , ∀Ï„, ∀m, ∀(t 1 , t 2 , …, t m ) . Where p(t 1 + Ï„, t 2 + Ï„, …, t m + Ï„) represents the cumulative distribution function of the unconditional (i.e., with no reference to any particular starting value) joint distribution. A process {X t } is said to be strictly stationary or strict-sense stationary if Ï„ doesn’t affect the function p. Thus, p is not a function of time. A time series {X t } ...