You asked
In November 2017, Consumer Price Index (CPI) went up to 3.1% (from 3% the previous month), but Retail Price Index (RPI) went down to 3.9% (from 4%) and RPIX went down to 4% (from 4.2%).
What factor(s) explain(s) why RPI and RPIX went down while CPI went up?
What factor(s) explain(s) why RPI is lower than RPIX?
We said
Thank you for your query.
The factors contributing to the narrowing of the difference between RPI and CPI were mainly house depreciation (which is included in the RPI, but not in the CPI), air fares (due to differences in the weight applied to this component in the RPI and CPI) and motor fuels (CPI takes an average of prices over the month, whereas RPI takes prices for a specific date).
RPIX is RPI excluding mortgage interest payments (MIPs). The inflation rate for Mortgage Interest Payments was -1.2% in November 2017, acting to reduce the RPI rate in comparison to RPIX.