Answer: A lot more than you think.
Watch this 4-minute video from WSJ to learn more
Here is some detail on the research study (from the Verge), which found with could find with eerie precision, who a person is just based on three anonymous credit card purchases:
The authors started with three months of credit card transactions from 1.1 million people, provided by an unnamed bank in an unnamedOECD country (the lead author, Yves-Alexandre de Montjoye of MIT, wouldn’t get more specific). They randomly pulled out a few single purchases for each person, then put the entire set in an anonymized database, removing details like names or bank account numbers. The database only gave prices within a range, since knowing that someone spent exactly $3.21 at Starbucks could give them away almost immediately. And researchers dropped super-high payments — more than $22,000 — for the same reason.
But this proved a minor stumbling block. When the authors mapped locations, dates, and prices of someone’s non-anonymous purchases against the whole database, it was usually easy to find a single, unique pattern. With three points or more, it was virtually a certainty. “You bought a coffee at that coffee shop, and you bought jeans at that shop, and then you bought a pizza,” says de Montjoye, by way of example. There’s a 94 percent chance that you’re the only person who did so.