By ALEX MINDLIN
Published: June 18, 2007
Tiny files called “cookies” are the lifeblood of online advertising. When a computer visits a site or sees an ad for the first time, the site’s server slips a cookie onto the visitor’s hard drive, identifying the computer in future dealings with that site or ad network. Cookies let online hosts determine the number of unique visitors they reach, a key metric for advertisers.
But this system of measurement has a well-known flaw: users are prone to delete their cookies, either manually or by using antispyware programs. Users who delete a cookie are eventually given a new one by that cookie’s issuer, meaning that they are often counted as unique visitors, inflating the numbers at host sites.
In a recently released study, the research firm comScore examined the fate of two commonly issued cookies across a panel of roughly 400,000 computers last December. It found that only 7.1 percent of the computers deleted their cookies four or more times over the month. But these “serial deleters” accounted for a grossly disproportionate share of the servers’ apparent traffic, receiving 35.3 percent of the total number of cookies observed.
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