NPD may be more accurate, but what do you prefer? Just for NA now.
1.
Weekly tracking of 200 games + hardware, over all of North America. Fairly accurate. Tracks all software, does not ignore some sort.
2.
Monthly tracking of 10 games + hardware, in USA. Very accurate, ignores some software.
3.
20.000 games, daily tracking, in NA, next to no accuracy at all.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS