comScore: Cookies Inflate Online Audiences By As Much As 150%
A study by comScore finds that measuring a web sites audience using cookies can lead to counts of "unique visitors" by a factor of more than 10. This is because many users clear their cookies on a regular basis, resulting in new cookies being installed on subsequent visits to a web site.
...Web site server logs that count unique cookies to measure unique visitors are likely to be exaggerating the size of the site’s audience by a factor as high as 2.5, or an overstatement of 150 percent.
Industry experts said:
- Mohanbir Sawhney, McCormick Tribune Professor of Technology at the Kellogg School of Management, Northwestern University. “To measure audiences more accurately, it is important to link visits to unique individuals, not unique cookies. As privacy programs become more entrenched, cookie-based audience counts will get even more unreliable.”
- David Verklin, CEO of Carat Americas. “Cookies are just not an accurate enough method to calculate site visits and upon which to base audience metrics.”
- Bill Cook, PhD, Senior Vice President for Research and Standards at the Advertising Research Foundation “Reach and frequency metrics are the cornerstone of any media plan, and given the size of the discrepancies that can occur when counting cookies instead of people, the study underscores the importance of panel-based measurement.
These findings mean that there will be downward pressure on revenues for top online sites as ad agencies seek improved collection of audience numbers. This adds a further issue for web publishers who already have problems in audience measurement because of RSS, AJAX, and caching technologies.
[The study is very self-serving because comScore offers services to count audiences. "Measuring the digital world" is its tagline.]