I don't think people realise how nuts 7m sold by the end of march would be. Consider the fact they're unlikely to have broken 400k for the March NPD (which in itself would be a pretty sizeable increase.) And Japan's Sales at less than 160k for the same period. That means "rest of the world" would have to be 440k? Extremely doubtful. This also assumes that PS4 even reaches 400k in the US this month.
PS4 sells 4.2 million WW as of Dec. 28th
PS4 sells 5.3 million WW as of Feb. 8th
PS4 sells 6 million WW as of March 2nd
January 2014 NPD - PS4 sells ~271K
February 2014 NPD - PS4 sells ~ 269K
Adjusting for 4 weeks instead of 5, we get approximately 880K untis sold WW between Dec. 28th and Feb. 8th, with ~271K units sold in the US in that timeframe, the US only accounted for approximately 31% of WW sales during that period
Between Feb. 8th and March 2nd, approximately 700K units were sold WW with approximately 202K units sold in the US [adjusted to 3 weeks instead of 4] which again puts the US at approximately 29% of WW PS4 sales
For the holiday season of 2013, PS4 sold 1138K [Nov.] + 865K [Dec.] which is 2003K out of 4200K WW at that time. Thus for the period of launch on Nov. 15 to Dec. 28th PS4 sold approximately 48% of its WW sales in the US
Using US LTD versus WW LTD as of March 2nd, we have 2543K out of 6000K which shows that 42.4% of PS4 sales are in US.
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Using the highest US ratio [48%], if PS4 sells 400K in the US in March we can expect 833K sales WW in the same time frame. It should be noted that this wouldn't account for Japan demand/sales or any other region added since Dec. 28th
Using the lowest US ratio [29%] we get 1379K sales WW for the March time period.
Using the US LTD versus WW LTD ratio [42.4%], we get 943K sold WW in that time frame although this would heavily favor the holiday months due to the unit sales
Using an average of the 3 separate time periods [36% = (29%+31%+48%)/3] US ratio we get 1111K PS4's sold WW in that time period
If PS4 sells 400K+ on March NPD I will be shocked if they don't reach 7M WW in that timeframe