The number of inferred individual people (filtered for spiders and robots), within a
designated reporting timeframe, with activity consisting of one or more visits to a
site. Each individual is counted only once in the unique visitor measure for the
Authentication, either active or passive, is the most accurate way to track unique
visitors. However, because most sites do not require a user login, the
most predominant method of identifying unique visitors is via a persistent cookie
that stores and returns a unique id value. Because different methods are used to
track unique visitors, you should ask your tool provider how they calculate this
A unique visitor count is always associated with a time period (most often day, week,
or month), and it is a “non-additive” metric. This means that unique visitors can not
be added together over time, over page views, or over groups of content, because
one visitor can view multiple pages or make multiple visits in the time frame studied.
Their activity will be over-represented unless they are de-duplicated.
The deletion of cookies, whether 1st party or 3rd party, will cause unique visitors to
be inflated over the actual number of people visiting the site. Users that block
cookies may or may not be counted as unique visitors, and this metric is handled in
different ways depending on the analytics tool used. Ask your tool provider how
blocked cookies are managed in their tool: it is important to understand how this
impacts other metrics with regard to these visitors.