Every month, we compare how much the UK's biggest supermarkets charge for a trolley packed full of essentials ranging from bread and milk to tea bags and toothpaste.
There's a regular price war between Aldi and Lidl, the two discounters, and often huge differences between the larger supermarkets.
You may be surprised by how the grocers rank when we compare them...
We track the prices of up to 200 branded and own-label products every day over the course of a month to work out the average cost per item, then add those averages up to get a trolley total for each retailer. We include special offers but not multibuys.
The contents of our trolley vary from month to month and might include anything from Andrex toilet paper and Lurpak butter to own-label bread and Heinz baked beans.
Our data is supplied by an independent price comparison company. Most of it is based on online prices, but for Aldi and Lidl (which don't sell their full ranges online) we send fieldworkers out into stores to see how much each item costs each day.
We rotate the products in the trolley to ensure we're comparing as many branded products as possible. This means the trolley totals aren't comparable between months or years.
At Which?, we’ve been analysing supermarket prices for years, but we only added Aldi and Lidl to our research in 2020.
Why? We used to only include branded products in our trolley. This had the advantage of ensuring that we were strictly comparing like with like. However, the downside was that, as Aldi and Lidl primarily sell own-label items with just a small supplementary range of branded goods, they couldn’t be included.
Many Which? members told us that they would find it helpful to be able to see how the discounters compared with the mainstream supermarkets. We listened to that feedback and have added own-label items to our trolley to enable us to widen the net of retailers we include.
We use experts to ensure that own-label products are as comparable as possible between the supermarkets, based on a range of factors including weight, quality and other industry data.