The behavior of the dollar against certain currencies appears to be
forecastable. In particular, the exchange rate appears to follow long swings -
it drifts upward for a considerable period of time, and then switches to a
long period with downward drift. Engel and Hamilton (1990) found that the
dollar/mark, dollar/pound and dollar/French franc exchange rates can be
described well by Hamilton’s (1989) Markov switching model.