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I had a discussion with a marketing teacher about datamining, and there was a ridiculous (da fuck you didn't think about that?) moment.
He said, that they do mine info about geographic areas using the sale cards. I asked what about people who don't want to provide name and location.
He looked at ne and said: that is ridiculously rare.
Then I realized, that he thinks that because he uses data only of people, who do have that sale card.
Plus, they are contaminating the shit out of that data. Since few people want that card, I have used friends card many times, and since they do sometimes even collect points for using it, I have been offered to use the cashiers card on a few occasions.