Video games often follow a predictable model of sales. Several factors must be taken into place before making any such model.
In the United States, NPD December accounts for nearly 25%, while October, November, and December account for 50% of the calendar year's revenue. Additionally, history shows that from January to October within any given year the data for a system's installed base is roughly a linear function. This is always subject to change if there is a price cut or a system seller game like Halo.[1]
The top 10 can account for 25-45% of the market (in dollars or units).[2]
The dropoff from #10 to #20 for a non-holiday month is around 60K to 100K. See for example, Feb 08 and April 08.
The grey market can be used to gauge supply issues. For certain cases, there is a strong correlation in monthly unit sales and grey market prices from auction sites like EBay.
North American console sales are usually at least twice as big as they are in Europe or Japan.[3] Compare Wii and PS2 sales in Japan and USA.