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MODULE CODE: MT4527
MODULE TITLE: Time Series Analysis EXAM DURATION: 2 hours EXAM INSTRUCTIONS: Attempt ALL questions. The number in square brackets shows the maximum marks obtainable for that question or part-question. Your answers should contain the full working required to justify your solutions. A formulae sheet is provided at the end of the exam. PERMITTED MATERIALS: Non-programmable calculator YOU MUST HAND IN THIS EXAM PAPER AT THE END OF THE EXAM PLEASE DO NOT TURN OVER THIS EXAM PAPER UNTIL YOU ARE INSTRUCTED TO DO SO. MT4527 May 2019, Page 1 of 12 1. (a) Explain in simple words what the state-space of a stochastic process Xt is. [1] (b) Consider a random walk Xt = Xt1 + "t, t = 0, 1, 2, . . ., with initial value X0 = 0. Here, "t is a white noise stochastic process. (i) Derive the expectation, E(Xt), for all t = 1, 2, 3, . . . [1] (ii) Derive the variance, V ar(Xt), for all t = 1, 2, 3, . . . [2] (iii) Derive the covariance, Cov(Xt, Xt+h), for all h = 0, 1, 2, . . .. Hence, derive the correlation Corr(Xt, Xt+h), for all h = 0, 1, 2, . . .. [3] (c) Consider a Simple Moving Average (SMA) estimator for the trend compo- nent of a non-seasonal model with trend. The choice of span q is ad hoc, and subject to the trade-o↵ between two of the estimator’s properties. Name those two properties. [2] 2. Consider a stationary AR(p) process with general form, Xt = 1Xt1 + 2Xt2 + · · ·+ pXtp + "t, t = 0,±1,±2, . . . where "t denotes a white noise stochastic process. Derive the Yule-Walker equations for the calculation of the autocorrelation sequence for the AR(p) process, including the initial conditions. [3] MT4527 May 2019, Page 2 of 12 3. Let "t denote a white noise process. Consider the AR(2) process {Xt} given by, Xt + 1 3 Xt1 2 9 Xt2 = "t. (a) Show that this stochastic process is stationary by considering and solving the relevant equation (x) = 0, where (x) represents the AR operator in a standard ARMA model. [1] (b) Substitute the general expression for the MA(1) process into the defining equation of the AR process, and show that the coecients of the MA(1) process satisfy a recurrence relation, specifying the recurrence relation and the initial conditions. [4] (c) Hence, calculate the coecients of the infinite MA representation. [4] (d) Assume without proof that, for L > 0, the minimum mean square error forecast xˆt(L), based at origin t, of Xt+L satisfies the equation Xt+L = ⌫0"t+L + ⌫1"t+L1 + . . .+ ⌫L1"t+1 + xˆt(L). Use this equation to show that xˆt(L) equals the conditional mean of Xt+L given the information available at time t. Show also that the corresponding conditional variance of Xt+L equals 2"
⌫20 + ⌫ 2 1 + . . .+ ⌫ 2 L1
. [4] (e) Suppose now that the random shocks are Normally distributed and that, after 80 observations have been seen, the point forecast of X82 made at origin t = 80 is xˆ80(2) = 60. Evaluate the 95% prediction interval for X82 made at the same origin, if the estimate of the white noise variance is s2" = 81. You may find the information below helpful: > qnorm(0.975) [1] 1.959964 [3] MT4527 May 2019, Page 3 of 12 4. A statistician is asked to model the changes in temperature (measured in the Farhenheit scale) during a chemical experiment. He has a time series x of 200 measurements taken every hour. For example, a measurement of xt = 10 describes a drop of 10 oF from hour t 1 to hour t. (a) The statistician plots the original time series (x) as well as the first (rx), the second (r2x) and the third (r3x) order di↵erenced series - see plot below. Decide based on the plot the likely minimum order of di↵erencing, necessary to obtain a stationary time series. Briefly justify your answer. [2] 0 50 100 150 200 −4 0 −2 0 0 Time x Original time series 0 50 100 150 200 −4 0 2 4 Time ∇ x First order differences 0 50 100 150 200 −2 0 2 4 Time ∇ 2 x Second order differences 0 50 100 150 200 −6 −2 2 6 Time ∇ 3 x Third order differences (b) The statistician decides to fit an ARIMA(0, 1, 2) model using R. The fol- lowing output is produced: Call: arima(x = temp, order = c(0, 1, 2)) Coefficients: ma1 ma2 0.5074 0.8856 s.e. 0.0351 0.0367 sigma^2 estimated as 1.143: log likelihood = -298.72, aic = 603.43 MT4527 May 2019, Page 4 of 12 The statistician wants to use this model for forecasting. Write down, without using any backwards operators, an explicit di↵erence equation for Xt+L, where L = 1, 2, 3, . . . . [2] (c) Recall that the minimum mean square error forecast xˆt(L) for Xt+L, based at origin t, equals the conditional mean of Xt+L given the information available at time t. Hence write down, without proof, a set of di↵erence equations giving (for L = 1, 2, 3, ..) the forecast xˆt(L) of Xt+L. [3] (d) Let the forecast origin be t = 200. The observed values for the temperature changes xt, for t = 198, 199, 200, together with the corresponding estimated random shocks ✏t, were as follows: day t xt ✏t 198 -22.24 0.036 199 -24.54 -0.600 200 -25.03 -0.220 Calculate the minimum mean square error forecast of the change in tem- perature for t = 202. [4] (e) Consider the ARIMA(0,1,2) model (1 B)Xt = (1 + 0.5B + 0.9B2)"t. Express the (Xt)t2Z process in random shock form. [4] MT4527 May 2019, Page 5 of 12 5. A (G)ARCH model was fitted to the first order di↵erenced series (rx) of the observations on the changes in temperature, as described in Question 4 above. Coefficient(s): Estimate Std. Error t value Pr(>|t|) a0 1.723e+00 3.335e+00 0.517 0.605 a1 1.519e-01 1.061e-01 1.432 0.152 a2 4.519e-02 2.806e-01 0.161 0.872 b1 9.293e-15 1.840e+00 0.000 1.000 Diagnostic Tests: Jarque Bera Test data: Residuals X-squared = 1.8717, df = 2, p-value = 0.3923 Box-Ljung test data: Squared.Residuals X-squared = 0.065631, df = 1, p-value = 0.7978 Answer the following questions: (a) Which type of (G)ARCH model has been fitted? [1] (b) Write down the volatility equation of the fitted model. [1] (c) Considering the fitted model, is the variance of the white noise process finite? If so, compute an estimate for this variance. [2] (d) Given the provided R output, comment on whether the fitted model is appropriate for the (rx) data. [3] MT4527 May 2019, Page 6 of 12 Formulae Sheet General solutions of first and second order homogeneous recur- rence relations • First order homogeneous recurrence relation uk+1 uk = 0 for k = 0, 1, . . . . Solution: uk = A k, where A is a constant • Second order homogeneous recurrence relation uk+2 + c1uk+1 + c2uk = 0 for k = 0, 1, . . . with auxiliary equation: x2 + c1x+ c2 = 0 Solution – when the roots of the auxiliary equation, 1 and 2, are distinct: uk = A1 k 1 + A2 k 2, where A1, A2 are constants – when the auxiliary equation has a double root : uk = (A1 + kA2) k where A1, A2 are constants MT4527 May 2019, Page 7 of 12 MT4527: Time Series - Solutions 1. (a) The state space of Xt is the set of values that the random variables Xt may take. [1] [EASY - bookwork] (b) (i) For a random walk Xt, without drift and initial value X0 = 0, Xt = "t + "t1 + ...+ "1