http://finance.yahoo.com/news/lowest-airfares-100038329.html

"We performed four sets of simultaneous searches on different days at different times, for a total of 372 queries. To check for merchants offering different prices to consumers based on their computer’s browser history, we had at least one tester use a browser with a robust history of searching flights (which would be typical of a comparison shopper) and another using a “scrubbed” browser, or one cleared of all of the “cookies,” or data files, that record previous web searches. (Consumers Union, the policy and mobilization arm of Consumer Reports, has seen evidence of pricing based on search histories in previous investigations, not only with airlines and hotels but also with books and shoes.)"