People always say they are afraid of Google and how much information it has on them. The truth is: people shouldn't be afraid of Google. They should be afraid of credit card companies.
I have access to your full order history. I know everything you ever bought with a credit card. And yes, there are a lot of studies done on credit card purchases.
Some years ago, someone wrote a paper claiming he could get the age, gende and race only from the credit card purchase history. It worked very well. Today, with your full purchase information, we can even "guess" your income range, number of dependant and even weigh. We have a statistical profile of every customer. We can even calculate the odds you eat at McDonald's today, considering you ate there once every X day. In 98% of the time, this model is very accurate.
One drawback is that it requires a lot of information. That is why it takes a few years and then, we are fully able to track you. In many cases, we compare the profile calculated from your purchase history to who you really are (and you thought they asked your income for credit validation) to further improve our models, and track fraud, most of all. It's so sophisticated that if you order products a person in your group never ordered, your card will get automatically locked.
Every time you use your credit card, you leave tracks. And none of it is private. Any police officer can get every purchase you ever made - and it can be used against you. There are many, many cases where credit card purchase history were used to prove DUI (you took a large tab at a bar) indirectly.