smooth(#) specifies the smoothing parameter to be applied. The default is 1600. Ravn and Uhlig (2002) have shown that the smoothing parameter should vary by the fourth power of the frequency
observation ratios, so that for annual data a smoothing parameter of 6.25 is recommended, while for monthly data a smoothing parameter of 129,600 is recommended. If one of these data
frequencies are evident in the data, the appropriate default pa will be used. If other data frequencies are in use, or if the data frequency has not been set in tsset, the default
smoothing parameter of 1600 will be applied. Specification of the smooth option will override default behavior.