Least Absolute Deviation Estimation for All-Pass Time Series Models

An autoregressive-moving average model in which all of the roots of the autoregressive polynomial are reciprocals of roots of the moving average polynomial and vice versa is called an all-pass time series model. All-pass models generate uncorrelated (white noise) time series, but these series are not independent in the non-Gaussian case. An approximation to the likelihood of the model in the case of Laplace (two-sided exponential) noise yields a modi ed absolute deviations criterion, which can be used even if the underlying noise is not Laplace. Asymptotic normality for least absolute deviation estimators of the model parameters is established under general conditions. Behavior of the estimators in nite samples is studied via simulation. The methodology is applied to exchange rate returns to show that linear all-pass models can mimic \non-linear" behavior, and is applied to stock market volume data to illustrate a two-step procedure for tting noncausal autoregressions.