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Alexa Internet, Inc. is a California-based subsidiary company of Amazon.com that is best known for operating a website that provides information on web traffic to other websites.

Alexa ranks sites based on visits from users of its Alexa Toolbar for Internet Explorer and from integrated sidebars in Mozilla and Netscape. In addition to their own statusbar extension, Sparky (released July 2007), there are several third-party extensions for Mozilla Firefox:

* SearchStatus shows Google PageRank and Alexa TrafficRank
* About This Site Firefox plug-in that shows metadata from Alexa TrafficRank.

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.[15]

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.