When calculating unique users your web analytics tool is trying to work out the number of people who come to your website, it works as described below:

  1. If you are utilising a JavaScript solution to deploy tags on your site like Google Analytics. Then when a user requests a page on your site or an item (such as a .pdf file) your analytics tool will set a unique cookie on your users browser.
  2. The cookie will remain on the browser, this cookie contains a unique and anonymous collection of numbers and characters, this is called a string. There is no personally identifiable information being collected (With many web analytics tools it is against the terms of service to collect any personally identifiable information).
  3. Each time a user now visits your website from that particular browser and computer, this cookie is also known as a persistent cookie id, will be used to recognise that this user has visited the site before and is therefore a returning user.
  4. Then when you run reports in your chosen web analytics platform for any given date range, your unique users total will be calculated by counting all of the persistent and unique cookie id's (a single count of the cookie id as the id may of been used for several sessions during the time period) during the given time period.

It is worth noting that there are some nuances when dealing with this particular type of data. The first thing we should take into consideration is that each unique user may not actually be a unique person in reality, if you have a computer at home think who else uses your computer to visit a site you have also been to? So it is important to understand that although the unique users metric is a fairly decent approximation of the number of unique people visiting your site, it is not by any means a perfect measure and should therefore be treated with an element of caution.

Secondly the unique users metric can be influenced by the browser itself, for example your browser may be set to not accept cookies (high privacy levels) or you may be browsing on Chrome using the incognito function.