Unique Browser (formerly also Unique Visitor) is an unique and valid identifier (= IP + User-Agent). [1]

This metric does NOT measure a person. Instead, it is a measure of a device through which a person interacts with a website or network, in common with all measurement software.

Formerly it was also called the Unique Visitor, but it was misleading as it made people believing that they measure by this way real people.

It is one of the IFABC metrics.

## Problem with sum of the Unique Browser

Very often mistake is to take the number of Unique Browser for a specific period (for example a month) and then to make a simple sum of monthly numbers; this sum does not mean anything.

example with the clients on a shop

Imagine to own a small grocery and you want to know how much clients do you have:

• Monday come Jane Dee (am) = 1 client
• Tuesday comes Jane Dee (pm) = 1 client
• Wednesday comes Jane Dee (am) = 1 client
• Thursday comes Jane Dee (noon) = 1 client
• Friday comes Jane Dee (pm) = 1 client
• Saturday comes Jane Dee (am) and John Do (pm) = 2 clients

(Sunday you are closed)

When measuring number of clients per day, your measurement starts at 00:00 and ends 23:59. On Monday you had 1 client. Then the next day you set counters to zero; and Tuesday you had 1 client again and so on and so far. When measuring number of clients per week, you start measure on Monday 00:00 and you stop on Sunday 23:59. Then you had Monday 1 client and Tuesday the very same client came you again. And it was always the same loyal client coming every day. Only Saturday came 1 different client.

In other words you have during the week 2 clients, Jane Dee coming every day and John Do coming once a week. (You might try to make a simple sum of number of clients per day (2+1+1+1+1+2=8) and believe that you have 7 clients, but it is just because you would count Jane Dee 6x and John Doo 1x.

If your clients keep the same behaviour, so Jane coming every day and John coming once a week, the number of your clients per month will be also 2, as well as the number of the clients per year.