I'm trying to forecast sales time-series data across a product line (in R). I can identify groups of products that have very similar seasonality profiles that I would like to apply to others in a couple of situations:
- Forecasting new items that have no history.
- Forecasting Individual products where I want to use an aggregate of multiple products to smooth out the seasonality.
In the past I have typically used something like a classic multiplicative decomposition and applied that to a subjective number of recent periods for the item I'm trying to forecast to determine a deseasonalized trend which I can then apply a model like exponential smoothing.
I would like to use STL to identify the seasonal components (allowing for seasonality shifts over time). But I can't figure out how to scale either the seasonal component from STL or my target product to combine them.
Is there a way to do this, or should I just use simple multiplicative decomposition? FYI, the data I'm currently working has very high seasonality, but I'm not sure that matters.