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Counting the online audience through the use of cookies may be overstating the size of the audience for websites, due to Internet users deleting cookies from their machines.

ComScore: Cookie Crunch Causes Crummy Counts

The usefulness of cookies has been debated virtually since the time they came into broader use online. Cookies are small text files stored on a person's computer when they visit websites that use them. The website can in turn use those cookies later to enable certain commerce functions, or to track their movements for analytic purposes.

ComScore said in a study of 400,000 home PCs in December 2006 that cookie-based measurements could be off by 2.5 times, or 150 percent. That's a serious overstatement with implications for advertising and audience measurement.

The regular deletion of cookies by some Internet users causes this inflation. ComScore's Magid Abraham said: "With just 7 percent of computers accounting for 35 percent of all (first party) cookies, it’s clear that a certain segment of Internet users clears its cookies very frequently. These ‘serial resetters’ have the potential to wildly inflate a site’s internal unique visitor tally."

Cookies aren't being deleted selectively, as one might think in the case of third-party cookies. Those tend to be used for tracking or analytics purposes, and come from separate domains than the ones an Internet user visited.

But those third-party cookies are being deleted at a 38 percent clip, only a little more often than the first-party cookies. Some people probably aren't seeing a difference between the two when they clean their browsers.

Telecom blogger Mark Evans questioned the aims of comScore's study: "Question #1: what is comScore suggesting: that cookies are a bad way to measure unique visitors, or that anyone not using comScore’s service is making a mistake?"

Venture capitalist and blogger Fred Wilson said webmasters can't rely on just their own analytics data:

You need third party data as well. That's not to say that third party data (primarily panel data) is perfect either. You have to triangulate between all the numbers to get a decent view of what's actually going on.

Like most things in life, having more than one source of information usually gives the best overall picture. The challenge is going to be finding a combination of analytics that satisfies webmasters and the advertisers that do business with them.