A few weeks back, Jupiter Research released a study that found, shockingly, that users are deleting cookies at such high levels that web measurement is in danger of being a pointless exercise. The report, "Measuring Unique Visitors: Addressing the Dramatic Decline in Accuracy of Cookie-Based Measurement" by Eric Peterson, was met with howls of derision and disbelief. Now, Peterson writes on his blog, Nielsen/NetRatings has corraborating research.


The Jupiter report found that:

• 17 percent of consumers delete cookies weekly, 12 percent monthly, and 10 percent daily -- behavior that "cripples sites’ ability to track users and make critical marketing measurements."
  
  
• 58 percent of users run antispyware software (which deletes cookies), 56% clear cache (which contains cookies), and 52% manually delete cookies -- making for a "grim outlook" for technology that uses cookies to track behavior.
  
  

Nielsen/NetTracker found that cookie deletion rates vary from 7% to 50%, according to site; and that deletions of cookies from Google come in at 25%. Operating from the assumption that more technically sophisticated users would be more able and likely to delete cookies, he writes:

If Google, a site used by a fairly generic audience based on their over 40% reach and overall popularity, suffers from cookie deletion at a rate of 25%, what might be going on at a site more geared towards technical sophisticates like PCWorld.com, Financial Times or the CNET family of sites?

The Nielsen study used a direct reporting mechanism by running software on subject's PCs, rather than survey sampling.

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