One important development in the recent time-series literature is the examination of non linear adjustment mechanisms. Much of the impetus for this interest stems from a large number of studies showing that key macroeconomic variables such as real GDP, unemployment, and industrial production display asymmetric adjustment over the course of the business cycle. For example, Neftci (1984), Falk (1986), DeLong and Summers (1988), Terasvirta and Anderson (1992), Sichel (1993), Beaudry and Koop (1993), Potter (1995), Ramsey and Rothman (1996) and Bradley and Jensen (1997) all support various forms of asymmetric adjustment in one or more of these variables.