A new study from comScore has questioned the validity of using cookie-based data to measure website audiences, claiming that unique visitor counts for websites may be overstating the actual visitor totals by as much as 150%.

The measurement group, which surveyed 400,000 home computers, found that three in ten US internet users regularly delete cookies from their computers.

The study found that 31% of US internet users deleted their cookies during the month, which means that each time these users return to a website they are counted as a new, unique visitor.

The study looked at cookies for an unnamed but 'prominent' US web property and a third party ad server, each of which reaches audiences of well over 100m per month.

It found:

  • An average of 2.5 first-party cookies were observed on each PC.
  • 31% of users deleted first-party cookies at least once during the month - the study found an average of 4.7 different cookies for the site.
  • Just 7% deleted cookies more than four times, but this group accounted for 35% of all cookies in the study - an average of 12.5 per user for the site in question.

The figures for third-party cookies - those left by an ad server in this case - show a similar pattern:

  • An average of 2.6 cookies were observed for each PC for the third party ad server being studied.
  • 27% deleted these cookies at least once, and their PCs contained an average of 5.5 cookies for the ad server.
  • 7% deleted cookies more than four times, and these users accounted for 38% of cookies. An average of 14.2 per user.

Most cookie deletion seems to be the work of security/anti-spyware software, with the survey suggesting that selective cookie deletion is not common - just 4% of users said they deleted third party cookies only.

Related reading:
Cookie deletion discrepancies take the biscuit
What will replace the page impression?

Graham Charlton

Published 17 April, 2007 by Graham Charlton

Graham Charlton is the former Editor-in-Chief at Econsultancy. Follow him on Twitter or connect via Linkedin or Google+

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Comments (2)


Jason Carrington

I am an Affiliate of many merchants who use my site as an electronic billboard to advertise their products and serrvices. Over several months the COOKIE thing right and wrong has plagued me. Some advice says eliminate cookies, however visitors to my sites click on to the virtual merchant sites where they can browse and place orders. They leave a cookie from my site at that merchant and when they buy a product the merchant records a sale and a commission to me. So it is critical that this cookie setting is done perfectly as I in E commerce am in a different status than other site users. I need this infoprmation as I have received thousands of impressions, and even a decent number of click thrus, but very few sales and commissions recognized nor recorded. I know that the correct handling of cookies is critical as if a visitor from my site does not bring a cookie to the merchant he does not have to pay me a commission. Appreciate all intelligent responses on this. Also I hear the same kind of debates on enabling Active X id others can explain this please. Does Active x negate cookies. I have both enabled. Jason Carrington jcarrington@austin.rr.com My site is about general diversified SHOPPING and my place is CARCOMMonlineWORLD shoppingCenter

over 11 years ago

Chris Lake

Chris Lake, CEO at Empirical Proof

Most affiliate schemes tend to work on a 'last click' basis, so you won't be credited with the sale unless your site is the last place a customer visits before buying from a merchant. Your cookie could be overwritten by another affiliate's site, for example.

I think affiliate marketing only really works where you have a site with one or more landing pages designed with the *sole* intention of converting visitors to customers. It is about reinforcing purchase intent, and making sure that the visitor buys there and then. Otherwise you leave it up to chance and don't get paid a penny for your efforts.

I've hosted affiliate ads on high traffic (500,000+ uniques per month) sites that aren't in any way product orientated landing pages, and know for a fact that it doesn't work. All you end up doing is giving away lots of free exposure to merchants - for that you need to be charging on a CPM basis.

You can't just whack up an affiliate banner on any old website and expect to generate a decent amount of sales, even if the traffic and CTR is good. In one case, we referred more than 9,000 visitors to a merchant in a few months, and only received 7 sales! Not a very good conversion rate, but more understandable when people were in 'browse' mode (reading news) rather than 'buy' or 'research' mode, and of course our cookies will have been overwritten / will have elapsed.

Amazon's affiliate programme really focuses minds in this respect, since its cookie duration is a miserly 24 hours!

over 11 years ago

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