Cookie Deletion: Why It Should Matter to Advertisers and Publishers

Earlier this month, comScore released a study on the impact of cookie deletion on audience measurement in Australia. The report follows in a series of cookie deletion studies that comScore has performed around the globe including in the U.S., U.K., and Malaysia, that analyse audiences’ cookie deletion behaviours and the implications this has on determining true site visitor counts.

What makes cookie deletion such an integral issue in audience measurement?

The key impact of cookie deletion is that a single PC user may be counted as multiple visitors by Javascript tagging-based Web analytics, i.e., Google Analytics. If a single user deletes her cookies twice during the course of one month and visits the same site four times throughout that time period, the user would register as three unique cookies in the site’s analytics data and be counted as three unique visitors to the site. Then there is the fact that many people use more than one browser software on a computer; each browser software keeps its own cookie database and the effect of cookie deletion is further multiplied. This all causes a big problem for site-server measurement solutions that don’t account for cookie deletion, with the potential to overstate a site’s audience many times over.

What is the difference between first- and third-party cookies?

Often cookies are referred to as first-party or third-party cookies and the difference between the two is important to understand. Cookies associated with a site delivering Web page content directly requested by the user are known as first-party cookies – in other words, cookies that are hosted and served by the same domain as the site that the user is visiting. Such cookies are typically used to directly improve the user experience with the site, and, to some extent, the user knowingly leverages these cookies when they intentionally navigate to the site.

Third-party cookies are those not served by the site that the user is visiting, but are embedded on the page.   This includes most of the free Web analytics solutions like those offered from Google, Yahoo, and many others. However a page may have many other cookies that are associated with objects delivered within the context of a larger Web page. These cookies may be served with advertising, embedded content, hosted-content, or rich media applications managed by a third-party domain. Third-party cookies may be set by intermediate activity occurring between rendered page views, such as a request for an advertisement that appears on the page that is served by a third party. Many applications of third-party cookies provide means for tracking activity across a broad network.

Cookie Deletion Across Markets – A Global Behaviour

ComScore has observed that cookie deletion is a behaviour that crosses borders. During an April 2010 study on cookie deletion behaviours across markets, comScore observed that across the markets observed (Asia Pacific, Australia, Malaysia, New Zealand, United Kingdom, France, Germany, United States, and Brazil), each market saw third-party cookies being deleted on between 20-40 percent of computers, and in each case the number of cookies per computer was generally more than three. In terms of first-party cookies, deletion rates were in excess of 20 percent in each case and the average number of cookies per computer was generally between two and three.

ComScore observed that an average computer in the Asia Pacific region had 4.2 third-party cookies, and 4.0 first-party cookies. Among the three Asia Pacific markets measured individually (Australia, Malaysia, New Zealand) in the study, all three saw first third-party cookie deletion rates hover around 35 percent, while Malaysians exhibited the largest percent of first-party cookie deletion at 33.5 percent of Web users.

So, why is cookie deletion such a big deal?

The implications of cookie deletion are far-reaching, affecting both site-centric analytics and ad-server analytics, and ultimately leading to inaccuracies for those choosing to rely solely on server-based unique user/browser data for weekly or monthly audience numbers.

Cookie deletion leads to the following inaccuracies in site-centric measurement when unique user/browser numbers are used as a proxy for unique visitors to a website:

• Overstatement of unique visitor counts
• Understatement of repeat visitor counts
• Understatement of conversion rates

Cookie deletion leads to the following inaccuracies in ad server measurement when unique user/browser numbers are used as a proxy for the count of the number of unique recipients of ads:

• Overstatement of reach
• Understatement of frequency

It’s important for publishers and advertisers to recognise the shortcomings of just site-server solutions to audience measurement. Accounting for cookie deletion is imperative to gain an accurate understanding of a site’s actual traffic – and that traffic in terms of actual people rather than just cookies and browsers.

Accurate, reliable measurement that both publishers and advertisers agree on is the cornerstone of building a vibrant digital advertising economy. At the end of the day, advertising has always been about how many eyeballs and the number of times they see the advertisers’ messages.