When considering the business process being reviewed you must consider the possibility of external influences, do you need to have the ability to scale up supply very quickly or is it usual to have a quiet season?
It is always worth considering that now is not the worst or best case. Your product may become disliked it may be found your supplier uses dangerous materials or their deliveries are occasionaly late. It is best therefore to examine the past to estimate future events as well as desiging in flexibility to systems.