5 min read
What a discount on your receipt actually tells you
Member prices, multi-buys and the regular price printed beside them. How to tell a real saving from a price that was raised first.
A discount line on a receipt is unusual among grocery data in that it records two prices at once: what the thing normally costs, and what you paid. Almost nothing else you can collect does that.
One line, two facts
You paid 59,00. You also now know that the regular price on that day was 69,00, which is a fact about the shop rather than about you.
Why the regular price is the interesting half
Over time the discount lines in your own history build a record of undiscounted prices at the dates you happened to shop. That is the baseline everything else is measured against.
Without it you cannot tell the difference between a product that has genuinely become cheaper and a product that is simply on offer this week. Both look identical in a naive average of what you paid, and the second one will mislead you into thinking you have found a bargain you can rely on.
Three kinds of discount, which are not equivalent
A straightforward price reduction
The item is cheaper for a period. Nothing is required of you. This is the only kind where the saving printed on the receipt is the saving you made.
A member or loyalty price
Cheaper in exchange for your purchase history being attached to an identity. Whether that is a good trade is a separate question, but it is a trade, and the receipt shows only one side of it.
A multi-buy
Three for the price of two, or a lower price above a quantity threshold. The saving is real only if you would have bought three anyway. Otherwise you have spent more money for more product, which is a purchase rather than a saving, and it is worth being honest with yourself about which one just happened.
Multi-buys on perishables deserve particular suspicion. The saving is calculated on what you took home, and the cost is calculated on what you ate.
Telling a real saving from a raised price
The way to test a promotion is against your own history, not against the sign. If a product has cost 59,00 for a year and the receipt shows a regular price of 69,00 with 10,00 off, the promotion is returning you to the price you were already paying.
This pattern is common enough to be worth checking on the items you buy most. It is also why a price history has to keep the regular price and the paid price as separate numbers rather than collapsing them into one. Once you average them together the evidence is gone.
A reasonable way to use promotions
The products where stocking up on offer genuinely pays are the ones that are non-perishable, that you buy on a predictable cycle, and that you have enough price history on to recognise a good price when you see one. Coffee, tinned goods, detergent, pasta.
For everything else, the honest position is that a promotion is a reason to choose between two things you were going to buy anyway, not a reason to buy. Tracking what you actually paid over a year tends to make that obvious faster than any amount of resolve.