Unique Visitors and Absolute Unique Visitors: What's the Difference?

Google Analytics provides detailed information on various aspects of a website, from traffic numbers to the source that brought them to the site (i.e. Search, direct, or referral).

It offers a comprehensive overview of site performance, but with so many statistical components it can sometimes leave you scratching your head.

Such is the effect of the frequently mistaken 'Absolute Unique Visitors' and 'Unique Visitors' sections within Analytics.

For many, these two distinct measures are interchangeable; a dangerous assumption. In fact, strikingly different results can be produced by both measures when analysing the same site withinthe same time-frame.

It is necessary therefore to understand the varying characteristics between these sections if one is to master the wealth of information offered up by Google Analytics.

Most simply put,' Absolute Unique Visitors' are first time visitors to the site, whilst 'Unique Visitors' are first time visitors to the site within the selected time-frame.

Although both track new visitors using cookies, which are attached to a user when he/she originally reaches the website, 'Absolute Unique' measurements also take into account whether or not the user has been to the site prior to the start of the set time period as defined in the date range box.

A good explanation is provided by Google in their description of “How Google Analytics Uses Cookies”. For the identification of unique visitors, Analytics tracks the activity of a visitor by recording their unique browser ID, which is provided via the _utma cookie.

For example, if one user visited a website using two different web browsers, such as Firefox and Internet Explorer, Google Analytics would class this as 2 unique visitors. However, if two people using the same computer and browser visit the site, this is registered as one unique visitor.

Put another way, the algorithm for determining 'Absolute Unique Vistors' is as follows: a first visit within a 30 day time frame adds 1 to the total of 'Absolute Unique Visitors' and 1 to that day's total. If the same visitor returns the next day, this does not show up in the 30-day total, but it does appear in the daily total.

A final aspect to be aware of, is the sampling effect of Google Analytics. For custom reporting or advanced segments within Analytics, rather than computing all of the required data – which can be a long and laborious process – GA cheats and takes only a sample of the information, before providing an estimated number based on that sample.

This process can have a notable effect on your results, and if absolute accuracy is what you're looking for, try to analyse data within time periods with fewer than 500,000 visits.

The crux of Google Analytics lies in the interpretation of the data it compiles, and in order to get the most out of the reporting process, it is important to be aware of subtle yet significant discrepancies such as this one.