Thanks to economics, your forecasts will be right only 30% to 40% of the time, as per this recent article over on BBC News that asks why do economists get it so wrong. Whether you care to admit it or not, all forecasts implicitly assume that the general economic condition will stay the same, since that determines not only how much money your potential customers will have, but how much they will be willing to spend. But since the foundation of the economy — humans, resources, wars, natural disasters, technology, etc — are in a constant state of change and flux, all of the models used to describe the economy are flawed.
Thus, your forecasts are only likely to be right at the macro level. Since nearly every economic forecast will be right at some point, every product line forecast will be right at some point, but like a broken clock, may only display the correct volume 0.13% of the time. If you have years of past behaviour, you’ll be able to create a good forecast at the macro (year) level, but it will get less and less reliable as the time period shrinks, no matter how much you throw into your model. That’s why you need an adaptive and flexible supply chain that allows for relatively quick replenishment — so you can ramp up production and distribution when you need to, but not have too much inventory on hand when you don’t.