You asked
Please could you answer the following questions about CPI and RPI calculations:
How many times have the CPI & RPI calculation been revised?
When was it revised?
What changes were made to the calculation, and why was it revised?
We said
Thank you for your request about revisions to the CPI and RPI.
As background, the revisions policy for consumer price inflation statistics is published on our website. The CPI can be revised and has been revised on a number of occasions since its launch in 1997. These were in:
- 1998, when errors in the 1996 and 1997 weights were corrected. Revisions were no greater than +/-0.1 percentage points on the headline rate.
- 2000, when the final version of the international classification system for analysing results was adopted and earlier indices were reworked on to the new basis. The recalculation resulted in occasional revisions (generally no more than +/- 0.1 index points) to some of the sub-indices and the overall index.
- 2006, when the CPI was re-referenced from 1996 to 2005 = 100, and the method of calculating derived statistics, such as rates of change, was changed from one based on published rounded indices to unrounded indices. There were widespread revisions.
- 2016, when the CPI was re-referenced from 2005 to 2015=100 though the inflation rates were unchanged through this.
The RPI is not a revisable index. When errors were discovered in 1986 and 1995, we published information on the size of the error but did not revise the series. Instead the relevant series were set to the correct level going forward, that is, we corrected the cause of the error so that it did not recur in future months. The 1986 problem followed the introduction of a new computer system and the headline rate should have been 0.1 percentage point higher for most months between February 1986 and October 1987. The 1995 issue was caused by the use of a seasonally adjusted house price index instead of an unadjusted figure and the headline rate should have been 0.1 percentage point higher in March and May 1995.
I am assuming in answering this that you are interested in revisions to the headline figures. Improvements to the way in which the indices are calculated are implemented each year and published either as methodology papers, as part of a paper describing changes to the sample (or basket) of goods and services used to calculate inflation or in open letters to the Bank of England.