VGC uses the same methods as Famitsu/Media Create/Dengeki for Japan, NPD for N.America and GfK/Chart-Track for PAL territories.

The difference is that VGC uses a much smaller sample of hard data (like 5% or less of the market) compared to the major trackers

Famitsu and Media Create both have about 65-70%.... I don't know about Dengeki
NPD has about 60% in the US, not sure about Canada, but we seem to have stopped getting free Canada data anyway.
GfK/Chart-Track have around 90% I think.

The sample is direct, exact sales data from retailers or distributors.... this is then extrapolated to count for the whole market (although I believe some of the numbers GfK and Media Create release are left unextrapolated). It is possible to be just as accurate with 5% as it is with 90%, but in cases of major discrepancies it is usually better to favour the major trackers data. But just keep an open mind as both Famitsu and Media Create track similar percentages of the market, but can have quite different weekly figures, espescially in difficult periods around Christmas/January when Japan sales are higher.

NPD, although it is often the one used to discredit VGC data is definately the least reliable of the lot, and particularly as it is the only tracker for the USA it is hard to say what methods thy use, and some of the more strange sets of data from it suggest that it may adjust data by changing the upcoming data rather than editing the previous data (at least as far as the freely released data goes) This would mean that the free monthly figures they release are not accurate even to their own sales data... but if NPD release LTD figures for anything they should be pretty accurate.

Ah. sorry.
The second one is meant to say Chart-Track.

So the European charts are the most accurate of all?

How come they hardly ever give numbers?