Although no longer approved by the Statistics Authority, the RPI has existed for over 100 years. Until relatively recently, it has been the main inflation index used for many purposes, such as pension increases and revaluation of index-linked gilts. We have looked at UK inflation rates (RPI alone, not CPI or CPIH) over single years and over periods with the same duration. In all cases, the measurement is from the end of December. While it may well be surprising to see higher averages over longer periods, that is because the 1970s were automatically included but the deviations are smaller.
Separately, we compare RPI with CPI and CPI with CPIH (since initial publication).