The Wii's sales compared to the PS2 vary from region to region.
Japan - The Wii started out a bit lower than the PS2, but has gradually met and slightly surpassed its sales.
America - The Wii started out about the same as the PS2, but started to overtake its weekly sales consistently by a small but ever-increasing margin. Currently, its sales are approaching the PS2's peak sales levels in 2002, about 4 or 5 months ahead of when the PS2 got to that point.
Europe - Overall, the Wii's sales match the PS2's sales almost perfectly, until about 3 months ago, when they started rising even higher than the PS2's had been.
Combined - Although not as useful since the PS2 launched later in America and Europe, it does show that the Wii's actual time on the market overall has had a greater impact than the PS2's. This is more a case of availability in all regions at the same time more than of it truly outdoing the PS2 astronomically.
Overall, the system is currently a bit above par with PS2's sales in each region. It has a key advantage over the system in combined sales due to a near-simultaneous worldwide launch, but its success on a region-per-region basis is generally in the "above PS2" bracket right now.
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