Traffic
Unique visitor
Also called unique user.
Unique visitor is the de-duplicated count of visitor identifiers in a named period. The period is part of the number.
How it is measured
Count each identifier once inside the window. A month of unique visitors is not the sum of thirty daily counts, because the same people appear on many days.
Tarsier keeps nothing across days, so a unique count over a week or a month is built from daily counts and will lean high. Treat longer windows as an upper bound.
Worked example
A museum site averages 900 unique visitors a day but 11,000 in a month, because most people come once. Adding 30 daily figures gives 27,000, which overstates the audience by more than double.
When the board asks for reach, the director gives 11,000 and adds the note that this is a 30-day count.
How it differs
A unique visitor is the de-duplicated number over a window. A visitor is the identifier behind it. New visitor and returning visitor split the unique count into the two ways someone can arrive.
Common errors
Adding up daily uniques. Ignoring the device split. Treating it as an exact headcount. Mixing windows between slides. Leaving bots in the denominator.
In practice
State the window beside every unique figure in a report. Choose one window for trend, such as 28 days, and use it every time.