[WayBack] What happens when a huge number of people share a single grocery store loyalty card? – The Old New Thing
An interesting discussion in the comments besides this interesting article observation:
What messes up their data analysis is when two people with different lifestyles swap cards. The system sees that somebody who used to buy yogurt and bulk brewer’s yeast is now buying potato chips and frozen pizzas, and it can’t figure out what is going on.
–jeroen