Quantcast’s privacy policy doesn’t reference the __qca cookie by name, but it hints as to what functions it performs with cookies in general. They’re quite vague about it, though, which is probably to give them the freedom to do what they want with the information they record.
The summary from Quantcast’s ‘log data’ section is that it may use your “computer’s IP address, pixel code, referring HTTP location, current HTTP location, search string, time of the access, browser’s time, any searches made on the applicable website, and other statistics” in order to “analyze Log Data from different websites and combine it with other non Personally Identifiable Information to produce the Reports that are made available on the Quantcast.com Site, to enable web publishers and advertisers to deliver audience segments that are appropriate for their products or services.”
This information was taken from
http://webmasters.stackexchange.com/questions/17912/what-does-the-quantcast-qca-cookie-do
Base on this information I think it can be classified as a category 2 cookie. Do you agree?