4.3 Moving Average Process MA(q)

Transcription

1 66 CHAPTER 4. STATIONARY TS MODELS 4.3 Moving Average Process MA(q) Definition 4.5. {X t } is a moving-average process of order q if X t = Z t + θ 1 Z t θ q Z t q, (4.9) where and θ 1,...,θ q are constants. Z t WN(0, σ 2 ) Remark 4.6. X t is a linear combination of q + 1 white noise variables and we say that it is q-correlated, that is X t and X t+τ are uncorrelated for all lags τ > q. Remark 4.7. If Z t is an i.i.d process then X t is a strictly stationary TS since (Z t,...,z t q ) T d = (Z t+τ,..., Z t q+τ ) T for all τ. Then it is called q-dependent, that is X t and X t+τ are independent for all lags τ > q. Remark 4.8. Obviously, IID noise is a 0-dependent TS. White noise is a 0-correlated TS. MA(1) is 1-correlated TS if it is a combination of WN r.vs, 1-dependent if it is a combination of IID r.vs. Remark 4.9. The MA(q) process can also be written in the following equivalent form X t = θ(b)z t, (4.10) where the moving average operator θ(b) = 1 + θ 1 B + θ 2 B θ q B q (4.11) defines a linear combination of values in the shift operator B k Z t = Z t k.

2 4.3. MOVING AVERAGE PROCESS MA(Q) 67 Example 4.4. MA(2) process. This process is written as X t = Z t + θ 1 Z t 1 + θ 2 Z t 2 = (1 + θ 1 B + θ 2 B 2 )Z t. (4.12) What are the properties of MA(2)? As it is a combination of a zero mean white noise, it also has zero mean, i.e., E X t = E(Z t + θ 1 Z t 1 + θ 2 Z t 2 ) = 0. It is easy to calculate the covariance of X t and X t+τ. We get (1 + θ1 2 + θ2)σ 2 2 for τ = 0, (θ γ(τ) = cov(x t, X t+τ ) = 1 + θ 1 θ 2 )σ 2 for τ = ±1, θ 2 σ 2 for τ = ±2, 0 for τ > 2, which shows that the autocovariances depend on lag, but not on time. Dividing γ(τ) by γ(0) we obtain the autocorrelation function, 1 for τ = 0, θ 1 +θ 1 θ 2 for τ = ±1, 1+θ1 ρ(τ) = 2+θ2 2 θ 2 for τ = ±2 1+θ1 2+θ2 2 0 for τ > 2. MA(2) process is a weakly stationary, 2-correlated TS. Figure 4.5 shows MA(2) processes obtained from the simulated Gaussian white noise shown in Figure 4.1 for various values of the parameters (θ 1, θ 2 ). The blue series is x t = z t + 0.5z t z t 2, while the purple series is x t = z t + 5z t 1 + 5z t 2, where z t are realizations of an i.i.d. Gaussian noise. As you can see very different processes can be obtained for different sets of the parameters. This is an important property of MA(q) processes, which is a very large family of models. This property is reinforced by the following Proposition. Proposition 4.2. If {X t } is a stationary q-correlated time series with mean zero, then it can be represented as an MA(q) process.

3 68 CHAPTER 4. STATIONARY TS MODELS Simulated MA(2) time index Figure 4.5: Two simulated MA(2) processes, both from the white noise shown in Figure 4.1, but for different sets of parameters: (θ 1, θ 2 ) = (0.5, 0.5) and (θ 1, θ 2 ) = (5, 5). Series : GaussianWN$xt Series : GaussianWN$xt55 ACF ACF (a) Lag Lag (b) Figure 4.6: (a) Sample ACF for x t = z t + 0.5z t z t 2 and (b) for x t = z t + 5z t 1 + 5z t 2.

4 4.3. MOVING AVERAGE PROCESS MA(Q) 69 Also, the following theorem gives the form of ACF for a general MA(q). Theorem 4.2. An MA(q) process (as in Definition 4.5) is a weakly stationary TS with the ACVF { σ 2 q τ γ(τ) = j=0 θ jθ j+ τ, if τ q, (4.13) 0, if τ > q, where θ 0 is defined to be 1. The ACF of an MA(q) has a distinct cut-off at lag τ = q. Furthermore, if q is small the maximum value of ρ(1) is well below unity. It can be shown that ( ) π ρ(1) cos. (4.14) q Non-uniqueness of MA Models Consider an example of MA(1) whose ACF is X t = Z t + θz t 1 1, if τ = 0, θ ρ(τ) = if τ = ±1, 1+θ 2 0, if τ > 1. For q = 1, formula (4.14) means that the maximum value of ρ(1) is 0.5. It can be verified directly from the formula for the ACF above. Treating ρ(1) as a function of θ we can calculate its extrema. Denote Then f(θ) = θ 1 + θ 2. f (θ) = 1 θ2 (1 + θ 2 ) 2. The derivative is equal to 0 at θ = ±1 and the function f attains maximum at θ = 1 and minimum at θ = 1. We have f(1) = 1 2, f( 1) = 1 2. This fact can be helpful in recognizing MA(1) processes. In fact, MA(1) with θ = 1 may be uniquely identified from the autocorrelation function.

5 70 CHAPTER 4. STATIONARY TS MODELS However, it is easy to see that the form of the ACF stays the same for θ and for 1. Take for example 5 and 1. In both cases θ 5 1 if τ = 0, 5 ρ(τ) = if τ = ±1, 26 0 if τ > 1. Also, the pair σ 2 = 1, θ = 5 gives the same ACVF as the pair σ 2 = 25, θ = 1, 5 namely (1 + θ 2 )σ 2 = 26, if τ = 0, γ(τ) = θσ 2 = 5, if τ = ±1, 0, if τ > 1. Hence, the MA(1) processes X t = Z t Z t 1, Z t iid N(0, 25) and X t = Y t + 5Y t 1, Y t iid N(0, 1) are the same. We can observe the variable X t, not the noise variable, so we can not distinguish between these two models. Except for the case of θ = 1, a particular autocorrelation function will be compatible with two models. To which of the two models we should restrict our attention? Invertibility of MA Processes The MA(1) process can be expressed in terms of lagged values of X t by substituting repeatedly for lagged values of Z t. We have The substitution yields Z t = X t θz t 1 = X t θ(x t 1 θz t 2 ) Z t = X t θz t 1. = X t θx t 1 + θ 2 Z t 2 = X t θx t 1 + θ 2 (X t 2 θz t 3 ) = X t θx t 1 + θ 2 X t 2 θ 3 Z t 3 =... = X t θx t 1 + θ 2 X t 2 θ 3 X t 3 + θ 4 X t ( θ) n Z t n.

6 4.3. MOVING AVERAGE PROCESS MA(Q) 71 This can be rewritten as However, if θ < 1, then n 1 ( θ) n Z t n = Z t ( θ) j X t j. j=0 j=0 ( ) n 1 2 E Z t ( θ) j X t j = E ( θ 2n Zt n) 2 0 n and we say that the sum is convergent in the mean square sense. Hence, we obtain another representation of the model Z t = ( θ) j X t j. j=0 This is a representation of another class of models, called infinite autoregressive (AR) models. So we inverted MA(1) to an infinite AR. It was possible due to the assumption that θ < 1. Such a process is called an invertible process. This is a desired property of TS, so in the example we would choose the model with σ 2 = 25, θ = 1 5.

7 72 CHAPTER 4. STATIONARY TS MODELS 4.4 Linear Processes Definition 4.6. The TS {X t } is called a linear process if it has the representation X t = ψ j Z t j, (4.15) for all t, where Z t WN(0, σ 2 ) and {ψ j } is a sequence of constants such that ψ j <. Remark The condition ψ j < ensures that the process converges in the mean square sense, that is E ( X t n ) 2 ψ j Z t j 0 as n. j= n Remark MA( ) is a linear process with ψ j = 0 for j < 0 and ψ j = θ j for j 0, that is MA( ) has the representation X t = θ j Z t j, j=0 where θ 0 = 1. Note that the formula (4.15) can be written using the backward shift operator B. We have Z t j = B j Z t. Hence Denoting X t = ψ j Z t j = ψ(b) = ψ j B j Z t. ψ j B j, (4.16)

8 4.4. LINEAR PROCESSES 73 we can write the linear process in a neat way X t = ψ(b)z t. The operator ψ(b) is a linear filter, which when applied to a stationary process produces a stationary process. This fact is proved in the following proposition. Proposition 4.3. Let {Y t } be a stationary TS with mean zero and autocovariance function γ Y. If ψ j <, then the process X t = is stationary with mean zero and autocovariance function γ X (τ) = k= ψ j Y t j = ψ(b)y t (4.17) ψ j ψ k γ Y (τ k + j). (4.18) Proof. The assumption ψ j < assures convergence of the series. Now, since E Y t = 0, we have ( ) E X t = E ψ j Y t j = ψ j E(Y t j ) = 0 and E(X t X t+τ ) = E = = [( k= k= ψ j Y t j )( k= ψ j ψ k E(Y t j Y t+τ k ) ψ j ψ k γ Y (τ k + j). ψ k Y t+τ k )] It means that {X t } is a stationary TS with the autocavarianxe function given by formula (4.18). Corrolary 4.1. If {Y t } is a white noise process, then {X t } given by (4.17) is a stationary linear process with zero mean and the ACVF γ X (τ) = ψ j ψ j+τ σ 2. (4.19)