MAT 3379 (Summer 2022) Midterm
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MAT 3379 (Summer 2022)
Midterm
Q1 (5 points) Let E(X) = 2, VaT(X) = 9, E(Y) = 0, VaT(Y) = 4, and the correlation between X and Y is ρ = 0.25
Calculate :
(a) Var(X+Y)
(b) Cov(X, X+Y)
Q2 (5 points) Find the mean function and the covariance function for each of the processes below. For each processes, determine if the process is stationary or not stationary.
(a) The time series {xt }, such that xt = θ+tet and {et } ∼ WN(0,σ2 ), where θ is a constant.
(b) The process {Yt }, such that Yt = Xt −Xt −1 ,and Xt comes from part (a).
Q3 (5points) Consider a process {Xt } , such that
Xt + 0.4Xt −1 − 0.21Xt −2 = Zt − 0.09Zt −2 .
{Zt } ∼ WN(0,σ2 )
(a) Does this model have redundant parameter? if yes, reduce the model.
(b) Is the process stationary, causal, and invertible?
(c) Does this process have an ARMA(p ,q) model? If , yes, give the values to p and q .
Q4 (6 points) Consider three random variables X1 ,X2 ,X3 is a sta- tionary process, such that µX = 0, γX (0) = 10, γX (1) = 3, γX (2) = 1
(a) Give the best linear prediction of X3 knowing = (X2 ,X1 ) (b) Give mean square prediction error (MSPE) of (a).
(c) Give the best linear prediction of 2+5X3 knowing = (X2 ,X1 )
Q5(2 points) Consider an AR(1) model
Xt − ϕXt −1 = Zt ,
where |ϕ| < 1 and Zt are i.i.d. random variables with mean 0 and vari- ance σZ(2) . Derive a linear representation for Xt , i.e. find the coefficients ψj in Xt =对 ψj Zt −j .
Q6 (3 points) Let {Zt } be independent random variables with mean 0 and variance σ Z(2) . Consider the model Xt = Zt − Zt −1 . Evaluate PACF α(2).
Note: You can use the formula from Lecture Notes that represents α(2) in terms of correlations. Then plug-in the correlations for MA(1) model.
2022-07-27