SteamSpy is a web-based analytics service that estimates and tracks sales data for games on the Steam platform. Unlike official sales figures (which Valve rarely releases publicly), SteamSpy uses an algorithm to collect public Steam user profiles and estimate how many people own a particular game, how much time they spend playing it, and regional distribution.
SteamSpy’s accuracy plummeted overnight. The sample size dropped from millions of profiles to only those who voluntarily set their profiles to public (a much smaller, potentially biased sample). steamspy