The residuals from a model ∇???????? = (1 − 0.6????)???????? fitted to a series of ???? = 82 observations yielded the following residual autocorrelations:

???? ????????(????̂) ???? ????????(????̂)

1 0.39 6 −0.13 2 0.20 7 −0.05 3 0.09 8 0.06 4 0.04 9 0.11 5 0.09 10 0.02

(a) Plot the residual ACF and determine whether there are any abnormal values relative to white noise behavior.

(b) Calculate the chi-square statistic ????̃ for lags up to ???? = 10 and check whether the residual autocorrelation function as a whole is indicative of model inadequacy.

(c) What modified model would you now tentatively entertain, fit, and check?