Even if you haven't handed over your details and product preferences through a loyalty scheme, it's likely you have used a debit or credit card to pay for your shopping at some point – and this is another way that the supermarkets can track what we buy.
"We know that an anonymised card number paid for a particular basket of groceries one week and how much was spent with the same card number the following week," says a
Morrisons spokesperson. "It means we know when customers are lapsing because we won't see their card for a week. We use it to measure the effectiveness of promotions and events."
When asked whether its customers give permission for their card numbers to be tracked in this way, the supermarket says customers "would only need to opt in" if Morrisons intended to send them any form of communication.
"All the large grocers track payment cards in this way," says Matthew Harrop at data analysis firm
emnos. "All your till receipts are linked together using either a known customer identifier – or anonymously in the absence of a loyalty card – to analyse what you're buying and how loyal you are."