Why? Because every Web site you visit knows what operating system you're using. It receives that information automatically, along with other data that can be used to uniquely identify your machine, such as your IP address. (That's the semi-anonymous part I mentioned above.) Add profile data gleaned from behavioral tracking cookies, and the information they can use to decide what prices to show you becomes much richer. If you've provided your phone number or email address, they can add even more info about you, including data from public records databases and your offline purchase history.
But Web sites don't need to know your name to determine what type of consumer you are. All they need to know is which demographic “buckets” you fit into. You'll never know you're being shown a different deal than your next door neighbor. And the data they use to make these decisions is not at all obvious.
2. You are more than the sum of your “Likes”
A few months ago a group of Cambridge University researchers published the results of an experiment using a Facebook app that tracked everything its users “Liked” on the social network. They surveyed the app's users as to their demographic information, then plotted correlations between who they said they were and what they did on Facebook.
Using just Facebook Likes, the researchers were able to determine with impressive accuracy a person's gender, age, race, sexual tendencies, political leanings, drug habits, and more. Some of the correlations were, frankly, strange: People who like curly fries tend to have higher IQs. Some made intuitive sense: Men who like the TV show “Glee” are probably gay. And some were both: Fans of the death metal band Slayer were more likely to smoke cigarettes.
This is what data mining is about – making correlations between two or more seemingly unconnected bits of information. It's how tracking companies like ToneFuse know that John Lennon fans are more likely to own cats, or that Rhianna fans are good candidates for buying a cut-rate cruise. It's how the FBI and other three-letter agencies are hoping to track crooks and terrorists before they strike.