Given observations X1 = 1, X2 = 1.5, X3 = 1.2 and X4 calculate the residuals at times t = 1 and t = 2. (b) Consider a SARMA(1,0) × (1,1)4 process. State the condit on the model parameters so that the process is stationary. (c) A researcher attempts to fit a zero-mean ARMA(1, 1) model time series of T = 100 data points. The analysis gives that 100

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Q3 (a) Consider the ARMA(2, 1) model
X; = X,-1 -X1-2+ €4 + &4-1
%3D
Given observations X1 = 1, X2 = 1.5, X3
= 1.2 and X4 = 2,
[3]
calculate the residuals at times t = 1 and t = 2.
(b) Consider a SARMA(1,0) × (1, 1)4 process. State the conditions
on the model parameters so that the process is stationary.
[3]
(c) A researcher attempts to fit a zero-mean ARMA(1, 1) model to a
time series of T = 100 data points. The analysis gives that
100
Ele - ē)? = 50.5
t=1
where e, is the estimated residual at time t, and ē = E et/100.
This sum of squares is found to be 48.5 when attempting to fit
a zero-mean ARMA(2,2) model at the same data. Which of the
two models, ARMA(1, 1) and ARMA(2,2), provides a better fit
to the data according to the AIC criterion?
You are reminded of the formula AIC = TIn(s) + 2k from the
course notes with appropriately defined terms. You may also use
some of the following logarithms: In(50.5) = 3.92, In(48.5) = 3.88,
In(95) = 4.55, In(97) = 4.57, In(99) = 4.60, In(100) = 4.61.
[5]
Transcribed Image Text:Q3 (a) Consider the ARMA(2, 1) model X; = X,-1 -X1-2+ €4 + &4-1 %3D Given observations X1 = 1, X2 = 1.5, X3 = 1.2 and X4 = 2, [3] calculate the residuals at times t = 1 and t = 2. (b) Consider a SARMA(1,0) × (1, 1)4 process. State the conditions on the model parameters so that the process is stationary. [3] (c) A researcher attempts to fit a zero-mean ARMA(1, 1) model to a time series of T = 100 data points. The analysis gives that 100 Ele - ē)? = 50.5 t=1 where e, is the estimated residual at time t, and ē = E et/100. This sum of squares is found to be 48.5 when attempting to fit a zero-mean ARMA(2,2) model at the same data. Which of the two models, ARMA(1, 1) and ARMA(2,2), provides a better fit to the data according to the AIC criterion? You are reminded of the formula AIC = TIn(s) + 2k from the course notes with appropriately defined terms. You may also use some of the following logarithms: In(50.5) = 3.92, In(48.5) = 3.88, In(95) = 4.55, In(97) = 4.57, In(99) = 4.60, In(100) = 4.61. [5]
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