Problem 1. Let c, t 1,...,n be the cmort tine series from the astsa package. Here n = 508. Let z = Vc be the first differenced time series. (a) Plot both c and z. Discuss whether differencing seems a reasonable transformation for cmort. What is the length of r? (b) Show the sample ACF and PACF plots for rt. Is an AR(1) model a reasonable identification for r,? Why?

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Chapter2: Second-order Linear Odes
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PLEASE  ANSWER PART (A) AND (B)

Problem 1. Let c, t = 1,...,n be the cmort time series from the astsa
package. Here n = 508. Let r, = Ve be the first differenced time series.
(a) Plot both ct and rt. Discuss whether differencing seems a reasonable
transformation for cmort. What is the length of r;?
(b) Show the sample ACF and PACF plots for r. Is an AR(1) model a
reasonable identification for r? Why?
(c) Fit an AR(1) model to z4, t = 1,...,508, using the method of conditional
ML, using
(i) the ar.mle function
(ii) the arima function
(iii) sarima: astsa function.
Make sure you can write out the fitted AR(1) model by pulling out relevant
information from each function's output.
(d) Is the AR(1) coefficient o significant? Discuss.
(e) How will you determine whether the residuals from the AR(1) fit are WN
or not?
(f) For each case in (c), construct L = 4 step-ahead forecasts for a given all
the past data, and get the 95% forecast intervals. Make sure you learn to
use the predict() function in R and the sarima.for() function in astsa.
(g) From (f), and using the relation between a, and c, write down the forecast
for c509 from the fitted AR(1) model. That is, what is C508 (1) = ĉ509?
(h) What ARIMA model does c; follow, when r; is an AR(1)?
Transcribed Image Text:Problem 1. Let c, t = 1,...,n be the cmort time series from the astsa package. Here n = 508. Let r, = Ve be the first differenced time series. (a) Plot both ct and rt. Discuss whether differencing seems a reasonable transformation for cmort. What is the length of r;? (b) Show the sample ACF and PACF plots for r. Is an AR(1) model a reasonable identification for r? Why? (c) Fit an AR(1) model to z4, t = 1,...,508, using the method of conditional ML, using (i) the ar.mle function (ii) the arima function (iii) sarima: astsa function. Make sure you can write out the fitted AR(1) model by pulling out relevant information from each function's output. (d) Is the AR(1) coefficient o significant? Discuss. (e) How will you determine whether the residuals from the AR(1) fit are WN or not? (f) For each case in (c), construct L = 4 step-ahead forecasts for a given all the past data, and get the 95% forecast intervals. Make sure you learn to use the predict() function in R and the sarima.for() function in astsa. (g) From (f), and using the relation between a, and c, write down the forecast for c509 from the fitted AR(1) model. That is, what is C508 (1) = ĉ509? (h) What ARIMA model does c; follow, when r; is an AR(1)?
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