What are VGChartz' sources for weekly US numbers?

[Dolla Dolla]

TheSource has posted a few news articles showing the difference in percentages between NPD and VGCharts. The first NPD after American charts went weekly was off by a large margin, but the most recent NPD saw the weekly charts much closer to what was reported. I imagine, as ioi collects more and more data from more sources, you will see more accurate representations. I'm certain TheSource will provide another breakdown to show the differences between VGC and NPD for May when the time comes.