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How accurate is it?

Such as does it remove characters that haven't been seen in a while or does it keep stacking up?
If it's seen a character more than once, and that character has leveled since last seen, does it update that character or does it create a new number for the person new level?
Etc....

Things like that.

It's fairly accurate given enough samples and read correctly, the data for population and suchlike are based on characters seen in the last 30 days so while it does not give you up to the minute information it will give you a very good idea of the population level, ratios and such from a server provided there are enough samples submitted for the last 30 days.

The character record is updated when new data comes in with new level/guild or something like that, each character has a history though so you can see back in time guilds and levels at specific times provided that samples were collected for the server at that time.

Excellent. Thanks bud.

The only possible interference with 100% accuracy I have noticed is when characters are moved to another server. They show up on both the old server and new server, however since out of 6 million subscribers this is probably a very small number it shouldn't skew the numbers too badly.

and they don't count on the old server after 30 days, there are actually other issues as well but no one is saying it's 100% accurate, it's as close as you can reasonably get without access to Blizzard's internal data though.