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Web & Internet Definitions

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Definition of Unique Visitor

A unique visitor is a statistic describing a unit of traffic to a Web site, counting each visitor only once in the time frame of the report. This statistic is relevant to site publishers and advertisers as a measure of a site's true audience size, equivalent to the term "Reach" used in other media.

The number of Total Visitors to a site divided by Unique Visitors results in the derived statistic "Average Sessions Per Unique Visitor" which tells a publisher how many times each Unique Visitor came to their site on average in the time frame of the report. Average Sessions Per Unique Visitor is equivalent to "Frequency" used in other media.

The Unique Visitors statistic can only be measured accurately in two ways with current technology:

1) by requiring all Visitors to log-in to the site, thereby capturing the identity of each Visitor on each visit, or

2) by placing a cookie on each Visitor's computer, writing the cookie ID to a database, and checking for the cookie on each Visitor's computer each time they visit.

Some log file analysis software products calculate a proxy for Unique Visitors by counting the number of visitors from unique IP addresses. This statistic is a poor measure as it is distorted by two main factors. Many consumers access the internet from networks that allow them to share a single IP address, which results in the IP address proxy undercounting Unique Visitors. There are also Internet Service Providers (ISP), such as AOL and cable modem providers, that use Dynamic Host Configuration Protocol (DHCP) which provide their customers with a different IP addresses for request they make from a site, thereby making one visitor look like many, thereby causing the IP address proxy to overcount Unique Users.

Many log file analysis products also use a proxy for Unique Visitors in which they count multiple visitors from the same IP Address if they don't visit the site within the same 30-minute window. This statistic results in a higher number than counting unique IP addresses, but is not equivalent to Unique Visitors as there is not guarantee that a visitor from the same IP Address outside the 30-minute window is a different visitor than a previous Visitor within the window.