In public discussions and the media, the worn-out trope-bromide is that there are no privacy issues with anonymization. As I used to engineer security and have followed many areas over the ensuing years, this always really annoyed me to see yet another Big Lie repeated over and over—such things have a way of becoming accepted by repetition.
So how many credit card transactions does it take to identify a person more accurately than fingerprints? Well, a whopping eleven (11). Which you might make in a few days, easily.
In other words, there is no such thing as anonymous shopping data. At least not without using good ’ol cash, or some new form of encryption-based money (but the government will never allow that, and even today BitCoin transactions are not fully anonymous, and the authorities are clamping down on it in regulatory fashion, so as to stomp out such offenses).
Similar results are found in other security research in other venues. So the next time you’re told your data will be shared “anonymously”, OPT OUT.