An experimental UK index tracking the lowest-priced versions of 30 everyday grocery items rose around 17% over the year to September 2022, the Office for National Statistics (ONS) reported on October 25. ONS also changed its rule for replacing products missing from the online data. For businesses using scraped prices, the distinction matters: changing the product being tracked can move the index without a price rise on the same product. [S1]

ONS said the increase was broadly similar to the roughly 15% rise in its official food and non-alcoholic drinks measure over the same year, but warned against a direct comparison. The experimental series covered just 30 items and combined prices differently. Its observations came from supermarket websites, not receipts. ONS had no sales or expenditure data showing whether consumers bought the lowest-priced products, and the online catalogue did not necessarily match what was available in local stores. [S1]

Vegetable oil’s lowest-price measure rose 65% between September 2021 and September 2022, but ONS urged extra caution with that estimate. Some records for the cheapest oils were missing; in one case, the researchers tracked a more expensive oil instead. That is not evidence that every bottle became 65% dearer, or that the entire increase was a data problem. A missing record is not proof of a stockout, either: ONS said changes to retailer websites had sometimes prevented its scrapers from collecting data. [S1]

The October release changed how replacement products qualified. Its 20% substitution threshold was now calculated on a rolling basis, using minimum and maximum prices in the current and previous month, to reduce volatility from missing data. This still left trade-offs. Once a product qualified as a substitute, it remained eligible regardless of later price changes; an expensive product admitted while discounted could therefore become an unsuitable replacement. The threshold could also miss a newly introduced cheap product priced far below the one already being tracked. [S1]

The revision reaches back into the earlier figures. On May 30, 2022, ONS had reported a 6.0% annual increase in its lowest-price index for April. The October release described that April annual increase as around 7% and warned that the new substitution method changed estimates from the previous publication. Putting May’s April reading beside October’s September reading as though they came from one unchanged series would mix methods and release vintages. [S1][S2]

For a retailer’s price dashboard, the practical implication is to distinguish changes for matching products from substitutions and missing observations. That does not supply a correction to ONS’s 17% estimate; it identifies what to investigate before calling an index movement a price change. ONS collected prices daily and said it could not go back and recollect observations missed by its scrapers. [S1]

Source documents

  1. Tracking the price of the lowest-cost grocery items, UK, experimental analysis: April 2021 to September 2022https://www.ons.gov.uk/economy/inflationandpriceindices/articles/trackingthelowestcostgroceryitemsukexperimentalanalysis/april2021toseptember2022
  2. Tracking the price of the lowest-cost grocery items, UK, experimental analysis: April 2021 to April 2022https://www.ons.gov.uk/economy/inflationandpriceindices/articles/trackingthelowestcostgroceryitemsukexperimentalanalysis/april2021toapril2022