Core RPI MoM

Core RPI MoM is a vendor label for the month-on-month % change in the Retail Prices Index (RPI), a UK retail inflation gauge. The Office for National Statistics (ONS), the UK statistics agency, publishes no series called Core RPI and gives no core exclusion definition. RPI tracks goods and services for private index households, defined as UK households excluding top earners, state-pension pensioners, institutions and visitors.

The official index began in January 1956 after an interim series from June 1947, with the published series set at January 1987 equals 100. It is not CPI or CPIH, 2 newer inflation gauges, not the inflation target and not a cost-of-living measure for excluded groups.

Collectors gather most prices on the Tuesday of index week, a working week near mid-month, with petrol set at 1 collection-date price. ONS issues results at 7.00am, usually on the 2nd or 3rd Wednesday after the reference month, often between the 13th and 21st. Dates appear in advance on the release calendar, and each release carries CPIH, CPI and RPI indices, changes, weights and driver notes. Once published, RPI is never revised to protect linked payouts, and errors above 0.1 points trigger a public correction notice.

Monthly moves reflect broad retail groups, with housing and clothing methods creating the largest gap to other gauges.

• Food: everyday groceries tracked with scanner checkout data.
• Housing: mortgage interest, Council Tax, depreciation and upkeep unique to RPI.
• Fuel and light: household energy costs inside the housing block.
• Clothing and footwear: apparel prices sensitive to averaging methods and collection practice.
• Motoring and travel: vehicle costs, fares and petrol priced at 1 date.
• Leisure, catering and household goods: services, meals and home items bought for consumption.

September 2023 data were published October 18, 2023. Annual RPI reached 8.9% with an index of 378.4, while RPIX stood at 7.6% with an index of 375.3. In August, RPI was 376.6 at 9.1% and RPIX was 373.7 at 7.8%, so mortgage interest held headline 1.3 points above the ex-mortgage gauge.

The index rose from 376.6 to 378.4, about 0.5% month on month on rounded indices. Yet the annual rate fell from 9.1% to 8.9% because the year-ago base was high.

1. Sample and collect: ONS prices about 760 items with about 180000 quotes around mid-month Tuesday, called index day. Basket, the item list, stays fixed within the year and refreshes each February.
2. Elementary averaging: Where detailed weights are missing, RPI averages price relatives with Carli, the mean of ratios, or Dutot, the ratio of mean prices. CPIH and CPI instead use Jevons, the geometric mean of ratios.
3. Higher aggregation: Higher indices are weighted averages of lower indices set to 100 in January, using spending weights in parts per 1000. Item weights draw on the Living Costs and Food Survey, a household spending survey, and admin sources.
4. Chain-linking: Each February, yearly January-based series link onto the long series based at January 1987 equals 100. Monthly chaining is avoided because Carli would overstate growth when prices bounce back.
5. Monthly change: Monthly change equals current index divided by previous index minus 1, times 100, using published rounded indices. Annual change compares the index with the same month 12 months earlier. For example, 378.4 divided by 376.6 minus 1 times 100 gives about 0.5% after rounding to 1 decimal place.
6. Seasonal adjustment: Headline RPI has no seasonal adjustment for regular seasonal patterns. Supplementary work studies seasonality but does not change the headline number.