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ECMT3150: Assignment
1. [Total: 24 marks] Note: Please append your R codes (as a separate .R le) for part (g) while you submit the assignment. Let Xi denote the log-price of a stock, Cherry Inc. (code: CRRY), by the end of trading day i, and let Xi := Xi Xi1; thus Xi is the log-return on trading day i (i.e., over period (i 1; i]). Assume fXig_i0 follows the AR(1) model: Xi = 0 + 1Xi1 + ui: (1) where ui iid normal with mean 0 and variance 2. Let fFigi0 be the natural ltration generated by fuigi0. (a) [2 marks] Express Xi in terms of Xi1 and ui. (b) [2 marks] Compute E(XijFi1). (c) [2 marks] Compute V ar(XijFi1). (d) [2 marks] What is the condition on 0 and 1 such that fXigi1 is a martingale di¤erence sequence? A trading strategy is de ned by figi0, where i is measurable with respect to Fi. Speci - cally, i represents the number of CRRY shares a trader buys at the start of day i. The log-return due to the trading strategy over period (0; T ] is given by rT = TX i=1 i1Xi. (e) [4 marks] Alice invested in a share of CRRY using a buy-and-hold strategy, with i 1 for all i. Compute E(rT ) and V ar(rT ) with 0 = 0 and 1 = 1. (f) [4 marks] Bob suggested another strategy, with i Xi for i > 0 and Compute E(rT ) and V ar(rT ) with 0 = 0 and 1 = 1. 1 (g) [8 marks] Carol suggested yet another strategy, with i 1fXi > 0g and 0 = 1. We want to evaluate the risk-return tradeo¤ of the proposed strategies using computer simulation. Start an R session, and set a random seed equal to the last 3 digits of your student ID.1 Then generate B sample values of rT (name them as r (1) T ; r (2) T ; : : : ; r (B) T ), and compute the sample mean and variance of rT as follows: rT = 1 B BX b=1 r (b) T ; se(rT ) = 1 B 1 BX b=1 (r (b) T rT )2: For the purpose of your simulations, set T = 63, 2 = 0:1, B = 1000. The Sharpe ratio, de ned as SR = rTse(rT ) , is a common measure of the risk-return tradeo¤. Trading strategies with higher SR are more preferred by investors. Complete the following table with SR values. Comment on the performance of the trading strategies under di¤erent scenarios. 0 1 Alice Bob Carol 0 1 0:01 1 0:01 1 0 0:9 0 1:1 2. [Total: 16 marks] LetM denote the mood of Mimi (h: happy; a: angry), and let W denote the weather (s: sunny; r: rainy). The joint probability distribution of M and W is given in the table below. The row and column sums are displayed in the last column and in the last row, respectively. p(m;w) M = h M = a W = s 0:4 0:1 W = r 0:2 0:3 (a) [2 marks] Compute P (M = a). (b) [2 marks] Derive the conditional distribution of W given M = a. Assume that, given m and w, your test score S follows a normal distribution with mean (m;w) := E(SjM = m;W = w) and standard deviation 5. The conditional mean function (m;w) is given in the table below: 1This is to ensure that your answers are replicable but di¤erent from those of other students. 2 (m;w) m = h m = a w = s 80 50 w = r 70 40 The passing score is 50 or above. (c) [3 marks] Compute the mean score E(S). (d) [3 marks] Given that Mimi was angry, what is the mean score you would get? (e) [3 marks] Compute the probability of failing the test. (f) [3 marks] Given that you failed the test, what is the probability that Mimi was angry? 3. [Total: 20 marks] Note: Please append your R codes (as a separate .R le) while you submit the assignment. Carol, an amateur economist, proposes the following time series model for unemployment rate: yt = 1 20 + p 3 2 yt1 1 4 yt2 + "t; (2) where "t iid N(0; 0:022) (normal distribution with mean 0 and variance 0:022). The time period is measured in number of quarters. (a) [3 marks] Show that the time series fytg generated by model (1) is stationary. (b) [3 marks] There is a stochastic cycle in the time series generated by model (1). Find its periodity in number of quarters. (c) [4 marks] Compute the ACF for the rst 3 lags, i.e., (1), (2) and (3). (d) [2 marks] Write an R program to simulate a sample path of fytg over 30 years. Set the initial values y0 and y1 to be y0 = 0:1 and y1 = 0:12. While simulating the random numbers for "t, set the random seed to be your last 3 digits of your student ID. (e) [2 marks] Plot the sample ACF and record its value for the rst 3 lags (the values can be retrieved from the acf command output stored as a list). Why are they di¤erent from your answers in part (c)? (f) [3 marks] Using the simulated sample path in part (d), estimate an AR(2) model using the R command arima. Write down the estimated model with the parameter estimates and their standard error. Also record the estimated variance of the innovations. (g) [3 marks] Using the simulated sample path in part (d) and the R package forecast, plot the point forecast and the con dence interval for each period over the next 5 years. Describe the short-run and long-run behaviour of the point forecast and the con dence interval.