What are VGChartz' sources for weekly US numbers?

[archmagus-21]

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

That's my point, though. I thought retailers don't give out weekly information anymore. That's why NPD is monthly now instead of weekly. How would VGChartz get weekly information?