What are VGChartz' sources for weekly US numbers?

[Alvar]

Avinash_Tyagi said:

Alvar said:
Its called educated guessing. Same for the Europe and Japanese numbers. It may be fun to create some discussion around, but don't bet your farm on any of these numbers to actually be correct.

 Actually its based on what retailers they do get data from and then extrapolation for the rest

Thats why its an educated guess, they extrapolate from a very small set of data. That doesnt make it any less of a guesstimate.