How do they come up with their stats, especially the demographics of the audience? Granted, anyone can tell that we're mostly dudes
, but how do they know age group of visitors?
Q: Where does Alexa get the traffic data?
Alexa gets its traffic data, including reach, page views and rank information, from a global panel of web users. The panel is used as a statistical sample of Internet usage to extrapolate overall traffic patterns and web usage information. The panel consists of Alexa Toolbar users and other sources web usage information.
Q: How reliable is the traffic data?
Alexa calculates traffic information for tens of millions of sites. However, the top 500 sites get almost 50% of all web traffic and compete for an increasingly smaller slice of the web traffic pie. Due to statistical limitations for the remaining millions of sites, we are unable to provide accurate traffic data for sites not within the top 100,000.
More info in the link below, although maybe not a definitive answer to your question
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