 The rationale is that this accounts for the fact that consumers will buy less of something if its price goes up, and more if its price goes down; it also ensures that if prices go up and then revert to the previous level, the CPI also reverts to its previous level (which is not the case with the calculation method used for the RPI).According to the ONS, this difference in averaging method is the largest contributing factor to the differences between the RPI and the CPI.The change in the CPI over the 12 months to August 2008 was 4.7%, while the corresponding figure for RPIX (which excludes mortgage interest) was 5.2% and that for RPI (which includes mortgage interest) was 4.8%.