BMAN71122: Time Series Econometrics Review Questions: I
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BMAN71122: Time Series Econometrics
Review Questions: I
1 Stationarity
1. Let {Xt } be a strictly stationary time series. Is it true that
● The random variables Xt are identically distributed for all t.
● The distribution of a random vector (Xt . Xt+h) is the same as the distribution of (X1 . Xh+1) for all t and h.
● {Xt } is weakly stationary.
Justify your answer.
2. Show that an i.i.d. sequence is strictly stationary.
3. Is any i.i.d. sequence is weakly stationary? Is it a white noise process? Justify your answer.
4. Let {Xt } and {yt } be independent weakly stationary time series. Is it true that the time series {Zt } defined by:
Zt = aXt + 8yt .
where a and 8 are known constants, is weakly stationary as well?
2 Time series models
1. Give an example of a white noise which is not an i.i.d. noise if such a time series exists.
2. Let {Xt } be a moving average processes of order 2 (MA(2)) defined by the equations:
Xt = Zt + 91 Zt − 1 + 92 Zt −2 .
where {Zt } ~ wN ╱0. 72、and 91 and 92 are real parameters.
● Prove that {Xt } is weakly stationary.
● Calculate the mean and variance functions of {Xt }.
● Derive the autocorrelation function of {Xt }.
3. A time series {Xt } is called autoregressive process of order 1 with mean u if the time series {Xt - u} is an AR(1) process (with mean zero). Assume now that {Xt } is a stationary series defined by equations:
Xt = c + φXt − 1 + Zt . t = o o o . -1. 0. 1. o o o .
where {Zt } ~ wN ╱0. 72、and c and φ are real parameters and lφl , 1.
● Calculate the mean function of {Xt }.
● Show that {Xt } is autoregressive process of order 1 with mean u.
● Derive autocorrelation function of {Xt }.
2023-05-18