BMAN71122: Time Series Econometrics Review Questions: II
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BMAN71122: Time Series Econometrics
Review Questions: II
1 ARMA models
1. Show that the two MA(1) processes:
Xt = Zt + Zt - 1 ; Zt ~ WN ╱0; 2、
Yt = Z~t + Z~t - 1 ; Z~t ~ WN ╱0; 2 2、;
where 0 < || < 1, have the same autocovariance function.
2. Let {Yt } be an AR(1) plus noise time series defined by:
Yt = Xt + Wt ;
where Wt ~ WN ╱0; W(2)、and {Xt } is the AR(1) process:
Xt = Xt - 1 + Zt ; Zt ~ WN ╱0; Z(2)、;
and 匝 [Zt Ws] = 0 for all s and t.
● Show that {Yt } is stationary and find its autocovariance function.
● Show that the time series {Ut = Yt 一 Yt - 1 } is 1-correlated.
● Conclude that {Yt } is an ARMA(1,1) process and express the three parameters of this model in terms of , W(2) and Z(2) .
3. Consider an ARMA(1,1) model defined by the equations:
Xt = Xt - 1 + Zt - 1 + Zt ;
where {Zt } ~ WN ╱0; 2、and || < 1.
● Define t = Zt - 1 + Zt and show that
o
Xt = j t -j :
j=0
● Obtain an MA(&) representation:
o
Xt = j Zt -j ;
j=0
where 0 = 1 and j = j - 1 ( + ) for j ≥ 1.
● Show that 匝 [Xt] = 0 for all t.
● Show that
X (0) = 2 1 + + ( + )
● Show that for h ≥ 1,
9X (h) = 62 (φ +1(u))(一φh - 1.
4. Let {Zt } ~ WN ╱0﹐ 62、. Check whether the following equations define stationary processes:
● Ⅹt = 0.3Ⅹt - 1 一 0.4Ⅹt -2 + Zt
● Ⅹt = 1.6Ⅹt - 1 一 0.65Ⅹt -2 + 0.05Ⅹt -3 Zt
● Ⅹt = 2Ⅹt - 1 一 5Ⅹt -2 一 2Zt - 1 + Zt
2023-05-18