Quantcast is a media measurement, web analytics service that allows users to view audience statistics for millions of websites. Quantcast Corporation's prime focus is to analyze the Internet's web sites in order to obtain accurate usage statistics by surfers from the USA. Like Alexa, Quantcast rates Web pages by ranks. Quantcast statistics always refer to the usage from the United States, therefore Alexa data and Quantcast data do not always show the same results. Quantcast does not require a toolbar to be installed upon one's web browser to obtain statistics.
Instead participating websites voluntarily insert Quantcast HTML code inside Web pages they wish to have included in statistics. This code allows Quantcast to keep track of the traffic directed towards those Web sites. Using this mechanism Quantcast can provide thorough details about Web pages created by participating publishers. Some of this information includes, for example, whether the Web page viewer is a male or female, whether the average viewer makes $30,000 USD annually or $100,000 USD annually, the age group of the viewer and the amount of U.S. homes the Web site reaches.
This information is provided by inference: comparing and correlating the information received from one participating publisher with another. The inferences are possible because the Quantcast code causes the user's browser to access Quantcast's servers, at which time they can log the user's IP address and information Quantcast places in cookies that are stored in the user's browser. The cookies significantly aid in making inferences. Quantcast also provides affinities revealing other popular sites that the average viewer browses. This is possible by tracking "referrer" information that is normally included as part of every HTTP request made by the user's browser. For instance a person browsing a page on Quantcast aimed towards music might also browse sites based on music downloads.
In February 2010, Quantcast was ranked 46th on Fast Company's 2010 list of the World's Most Innovative Companies and placed third in the Top 10 Most Innovative Companies on the Web, behind Facebook and Google respectively.
In July 2010 BBC News reported that a legal challenge has been launched in the US against a number of websites amid claims that they were engaged in "covert surveillance" of users through the use of a Quantcast Flash application to restore deleted cookies.
