omgwtfbbq's sounds like it would be the one used, especially by NPD and such.
My guess (until that post) would have been the set of weeks that have at least 4 days in that month (ex: January 28 - February 3 would be January, whereas January 29 - February 4 would be February). But that gets messy.
PS3: 5.51m/51w, avg 108,039/w (up 239)
360: 12.93m/102w, avg 126,764/w (up 625), leads PS3 by 7.42m (up 70k), avg lead 18,725/w (up 386)
Wii: 13.52m/51w, avg 265,098/w (dn 1,102), leads PS3 by 8.01m (up 90k), avg lead 157,059/w (dn 1,341)
If 360 sales stabilize, PS3 sales increases needed to pass 360 by...
01/08: (008w) +875.8%, 04/08: (021w) +344.4%, 07/08: (034w) +219.3%, 10/08: (047w) +163.5%
01/09: (060w) +131.8%, 04/09: (073w) +111.4%, 07/09: (085w) +098.1%, 10/09: (099w) +086.7%
If Wii sales stabilize, PS3 sales increases needed to pass Wii by...
01/08: (008w) +1072.%, 04/08: (021w) +498.4%, 07/08: (034w) +363.4%, 10/08: (047w) +303.1%
01/09: (060w) +269.0%, 04/09: (073w) +246.9%, 07/09: (085w) +232.6%, 10/09: (099w) +220.3%
If PS2 sales freeze, Wii sales increases needed to pass PS2 (as of Mar07, 108.4m) by...
2008: (008w) +4373.8%, 2009: (060w) +0496.5%, 2010: (112w) +0219.6%, 2011: (165w) +0116.9%
2012: (217w) +0064.9%, 2013: (269w) +0033.1%, 2014: (321w) +0011.5%, 2015: (376w) -0004.8%
At +0% it will pass it in 358w, the week ending September 19th, 2014, at an age of 409w (7y44w).
Current age of PS2: 7y37w.
Last update: Week ending November 3, 2007
