Product Phasedown Models in SAS FS

[T_Rey]

What is the best way to put a PHASEDOWN event in SAS FS?  We know it phased down to 0 and we know when it is supposed to go to zero.  Obvisouly this will be in the forecast horizon, not in the current date time frame.

[TammyJackson]

I assume that you need an event with a downward slope, and you need that event to be 0 past a certain point. Can you be a bit more specific? Is it 0 before a certain point? Is the slope linear or some other function?

[TammyJackson]

Is this the effect you wish to model?

[T_Rey]

Yes, using SAS FS and only in the forecast horizon

[TammyJackson]

If the dummy variable is 1 (constant) for all the observations in the entire historical period, that can* model a level (mean of the series), and that parameter estimate can be used to create a down ramp in the forecast. If the entire down ramp is in the forecast, then you are relying on your knowledge and experience to predict when the ramp will start and end and also the shape of the ramp. That is, there is no history to validate that any portion of the ramp is the correct model. However, if you have many other similar series, you might be able to use those as proxies to get an ideal of the shape of the ramp.

*The effect modeled by a dummy variable depends on whether or not the dummy is transformed (i.e. log) or differenced and whether the series is transformed or differenced. The effect should be appropriate for the type of model, additive or multiplicative.  Also, the dummy can be collinear with other effects in the model such as a mean term.