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Statistical Sciences 4861b/9861b Assignment #2 2022
发布时间:2022-03-05
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Statistical Sciences 4861b/9861b, 2022
Assignment #2
Assume the WN process et is iid with normal distribution for all the following questions
1. For an AR(2) process given by
(1 − 1.1B + 0.24B2)Yt = et
(a) Verify it to be stationary by finding x1 = 1/R1 and x2 = 1/R2 from the
characteristic equation and checking if |x1 | > 1 and |x2 | > 1.
(b) Use the Y-W equation ρt = 1.1ρt − 1 − 0.24ρt −2 to find ρ1 .
(c) Using ρ0 = 1 and ρ1 as starting values to to find ρt, t = 2, 3,..., 12. Note: use R for loop to find ρt, t = 2, 3,..., 12.
(d) Let ρt = b1R + b2R
, t = 0, 1, 2,.... Then
1 = ρ0 = b1R + b2R
= b1 + b2 .
Find another equation so that b1 and b2 can be solved. Compare to the com-
puted values ρt, t = 2, 3,..., 12 of (c). Are they matched?
(e) Plot the acf for this model and comment about its behavior.
2. Consider the following AR(2) models:
(i) Yt − 0.6Yt − 1 − 0.3Yt −2 = et . (ii) Yt − 0.8Yt − 1 + 0.5Yt −2 = et .
(a) Find the general expression for ρk .
(b) Plot the ρk, for k = 0, 1, 2,..., 10.
(c) Calculate σ by assuming that σ
= 1.
3. (a) Find the range of α such that the AR(2) process Yt = Yt − 1 + αYt −2 + et
is stationary.
(b) Find the acf for the model in part (a) with α = − 1/2.
4. (a) Simulate two MA(2) time series with n = 500 as follows. One is Yt = et + 0.3et − 1 − 0.4et −2 with et iid N(0, 1). Another one is Yt′ = e − 1.2e
− 1 − 1.6e
−2 with e
iid N(0, 0.25). Plot them and their corresponding sample acf. What is your finding?
(b) Find the theoretical acf of Yt and Yt′ respectively. What is your finding?
(c) Which time series has all roots outside of the unit circle and hence it is invert- ible?
5. Consider the MA(2) process Yt = et − 0.1et − 1 + 0.21et −2 .
(a) Is the model stationary? Why?
(b) Is the model invertible? Why?
(c) Find the acf for the above process.