WHAT is ALEXA

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Alexa is a site that provides service to calculate the traffic rankings by analyzing web usage, the traffic rank is based on three months of aggregated historical traffic data from millions of Alexa Toolbar users and data obtained from other sources of traffic data that varies, and is combined measure of page views and users (reach). As a first step, Alexa calculates the range and number of page views for all sites on the Web every day.
Page views measure the number of pages viewed by visitors to the site. Some displays pages from the same page made by the same user on the same day counted only once. Page Views per user numbers are the average number of pages viewed per unique user per day by visitors to the site. Changes in three months is determined by comparing the active site display page numbers with those from three months ago.

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source: www.alexa.com