Alexa ranks sites based on tracking information of users of its Alexa Toolbar for Internet Explorer and from integrated sidebars in Mozilla and Netscape.
There is some controversy over how representative Alexa’s user base is of typical Internet behavior. If Alexa’s user base is a fair statistical sample of the Internet user population (e.g., a random sample of sufficient size), Alexa’s ranking should be quite accurate. In reality, not much is known about the sample and possible sampling biases. Alexa itself notes several examples. A known source of bias is the self-selecting, opt-in nature of Alexa traffic tracking software installation, but the significance of this bias on rankings is not reported.
On April 16, 2008 many users reported dramatic shifts in their Alexa rankings. Alexa confirmed this later in the day with an announcement that they had released the new Alexa ranking system, claiming that they now take into account more data sources “beyond Alexa Toolbar users”.
On March 31, 2009 Alexa.com got a complete redesign and new metrics including: Pageviews per User, Bounce Rate, and Time on Site. In the following weeks they added new features including Demographics, Clickstream and Search Traffic stats.
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Tags: Alexa Credibility, Alexa Rank, Alexa Ranking, Alexa Toolbar