Webtrends Analytics 9 Adminstration Guide, November 2012
Cookie-dependent analytics solutions rely on the cookie as the method to identify one unique browsing session from another.
A number of business issues arise from third-party cookies being rejected or deleted on a regular basis:
Inaccurate Visitor Metrics: At its most fundamental level, if an Internet user has configured the browser security settings to automatically reject third-party cookies, that visitor is not properly counted in your web analytics results. As mentioned earlier, market estimates project this to be anywhere from 12% to 28% of Internet users on average.
Deceiving Retention Based Metrics: Taking this one step further, if John Doe visits your web site on May 15 and accepts the third-party cookie, he is recognized as a new visitor. If John then deletes all of his third-party cookies with his anti-spyware application on May 16th and returns to the site on May 17th, the analytics solution identifies John as a new visitor, since he no longer has the cookie on his computer. This would have an impact of under representing your retention based metrics such as your repeat visitor rate.
Inaccurate Conversion Metrics: Cookie deletion also has an impact on your conversion rate for new visitors versus repeat visitors.
Conversion rate = (conversion actions taken/number of visitors) X 100
where a conversion action is an action indicating visitor conversion, such as an order, and the visitors may be new or repeat visitors.
If the cookie is systematically deleted such as with an anti-spyware application, repeat visitor rates are under-counted and new visitor rates are going to be over-counted, skewing your conversion rate metric by which you analyze your site's overall effectiveness.
Unreliable Campaign, Search and Merchandising Reports: In addition to tracking the behavior of a visitor to the site in general, many analytics providers correlate visitor response and site interaction to a specific campaign, search engine or product in an attempt to understand precisely which campaign or merchandising offer inspired the Internet user to take an action; much of this information can rely on information stored in the cookie. If the cookie is rejected or deleted from the Internet user's browser, reports designed to identify latent or deferred conversion to a campaign or merchandising offer are misrepresented. It is also important to note that the longer that you track conversion to an individual marketing activity, the more likely it is that your metrics are inaccurate, as the likelihood the user deletes the third-party cookie increases.