It’s harder to count web visitors than was previously thought. According to, a single visitor can be counted many times if they regularly delete or block “cookies” from their computer.

Many web sites place a cookie, a small line of code on a visitor’s computer, when they first visit the site. This helps those sites collect information about the user and sometimes makes it easier for the user to navigate that site the next time they come to it. When the user deletes those cookies and later returns to that site, they are counted as a new user. This can and does happen over and over. It’s impossible to know how many “visitors” are simply the same people returning to the site later. This is particularly important to advertisers, since the rates they are charged are usually based on the number of visitors to that site or a section of the site.

The CEO of ComScore says his company did a survey of 400,000 PCs and found that a sizable number of users cleared the computer’s cookies on a regular basis. (We would be among those.) He estimated that many sites are then inflating the count of unique visitors by as much as ten times the true number.

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