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The standard testing procedures lot seasonal unit roots developed so far have been based mainly on time-invariant autoregressive integrated moving average (ARIMA) processes with AR polynomials involving seasonal differencing. One attractive alternative is to use periodic ARMA models. Convenient procedures are presented for testing for the presence of unit roots at the zero and seasonal frequencies in periodic time series. The limiting distributions are derived and tabulated, and simulation evidence illustrates the advantages of allowing for periodically. The tests are illustrated via applications to macroeconomic and ozone level data. [ABSTRACT FROM AUTHOR]

Several economists, notably Plosser (1978), Sargent (1978), and Wallis (1978) refuted the assumption that the seasonal component of endogenous variables in a dynamic economic model has almost all of its power restricted to what is termed seasonal frequency and its harmonics. In this article, a model with a closed-form solution is formulated to provide more insight into the arguments put forward by economists. It is concluded that univariate seasonal adjustment cannot be considered a harmless simplification of data without loss of information, neither for the interpretation of economic time series nor for regression analysis. [ABSTRACT FROM AUTHOR]