A recent report from the Center for Media Research that found the proportion between new and repeat visitors in cookie-based tracking tools was inaccurate.

The essential argument was that third-party cookies are consistently flushed by security programs because they are generally seen as superfluous or violations of security and/or privacy. According the research, 31% of all web browsers have their cookies cleared each month.

The consequence of this is that the visitors coming to the site are not identified based on their prior visits.

The whole study is here if you have a subscription, which is free.