But in fact, the main reason for the two indices rising at different rates is because they use different arithmetical methods for calculating average prices.
Instead, in March 2013 it will launch an additional index called RPIJ, a new version of the RPI index but using the same methods as CPI for calculating average prices.
That is far below what some of the cost projections coming out of the Pentagon suggest will be possible, reflecting divergent methods and assumptions used in calculating costs.
Unfortunately, there is no consensus on which methods and assumptions are most valid when calculating forward costs, especially costs incurred in the distant future.
For it is not taking account of several major poverty alleviation programs when calculating the number in poverty in the US and yet the calculations for the other countries do take all poverty alleviation methods into account.