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Measuring visitors, page views and actions on your Web site is finally settling down and becoming a manageable task.

As we get accustomed to the tools and the terminology, the prospect of tracking things online is no longer as frightening as it used to be. At least that was the case until November 20, 2003.

On that date, RedEye, a Web analytics vendor in London, released a little study it had done showing that everything you know is wrong. Here's what RedEye did, and what it found.

RedEye has two large clients with lots and lots of visitors: www.asda.com, the UK version of Wal-Mart, and www.willhill.com, an online gambling site (that's legal in the UK).

A majority of customers log in weekly to buy things or, even more often, to place wagers. When customers log in, you know a great deal about them. You know who they are, and through a session ID you know what they're up to. You can then compare their actions to their previous behavior.

Without the login, you have to tie that session ID to something else, typically an Internet protocol (IP) address or a cookie. We all thought that was fine.

Unfortunately, RedEye discovered that was not fine: “The results are staggering: the IP-based approach overestimated unique users by 7.6 times, whilst a cookie-based approach overestimated unique users by 2.3 times.”

How can this be?

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image of Jim Sterne

Jim Sterne (jsterne@targeting.com) is the founder and director of Target Marketing (www.targeting.com), founder of the eMetrics Marketing Optimization Summit, and the founding president and current chairman of the Web Analytics Association.