For a while now the general public has known that cookies are collected on your computer, they track some level of information about where you’ve been and what you did there and they can be easily deleted from the tools drop down of the browser toolbar. At first people were scared that cookies contained spyware and viruses. Now I think that is rare if ever possible. Its mostly just companies trying to figure out which ads worked and which didn’t. They drop the cookie when you visit and pick up that cookie’s random identifier number when you complete whatever they’re trying to get you to do. (buy something, apply to a job, complete a sign up form, stay on their site a specific amount of time, see which pages are most popular) This is what makes tracking ads on other sites possible so you can see what impact they have had on your site.
What some marketers forgot though, is that many people clear cookies for other reasons. Mostly for personal reasons when computers are shared in a household, at work or in libraries and other public places. People know that anyone can search the C drive and find the cookie folder and find their shopping, chatting or p*rn. So, they delete them. And often if they’re guilty of something.
comScore’s study stated that 7% of internet users delete their cookies at least once a month and it equates to about 30% of all cookies tracked. That may be lower than reality, but we will take it as a verifiable number. And that means that all tracking systems including theirs have a 30% surplus of traffic from these people who constantly clear everything out and start over clean. When I worked for an internet company we always noticed that there was a gap between what comScore reported for us and what we had as internal traffic stats. Now we know why. Take all 3rd party metrics reports (comScore, HitWise, Google Trends) with a grain of salt. Unless you know the methodology of the reporting and how things are processed, you don’t know how true the numbers really are.