If the operator increases its productivity faster than (conversely, slower than) the average firm, then the operator's retail prices (revenue) will need to decrease (conversely, increase) relative to the rate of inflation. Combining these two possible differences between the operator and the average firm in the economy, the operator's retail prices (revenue) should change at the rate of inflation, minus (conversely, plus) the extent to which its input prices inflate less than (conversely, greater than) the rate of inflation, and minus (conversely, plus) the extent to which the operator's productivity is expected to improve at a rate that is greater than (conversely, less than) the average firm in the economy. The above analysis identifies two things. First, the inflation rate, I, used in the price cap index represents the general rate of inflation for the economy. Second, the X-factor is intended to capture the difference between the operator and the average firm in the economy with respect to inflation in input prices and changes in productivity. That is to say, the choice of inflation index and of the X-factor go hand in hand. Some regulators choose a general measure of inflation, such as a gross national product price index. In this case, the X-factor reflects the difference between the operator and the average firm in the economy with respect to the operator's ability to improve its productivity and the effect of inflation on the operator's input costs. Other regulators choose a retail (or producer) price index. In these cases, the X-factor represents the difference between the operator and the average retail (or wholesale) firm. Lastly, some regulators construct price indices of operator inputs. In these cases, the X-factor reflects productivity changes of the operator. In most industries in the UK, estimation of a firm's efficiency is carried out by comparing regional monopolies and using a total factor productivity method. However, for telecommunications, Ofcom instead relies on international comparisons. In practice, the distinction between price-cap and rate-of-return regulation may be lost, as regulators may end up making implicit decisions on the acceptable real rates of return on capital employed in order to arrive at price limit determinations. This has been the experience in the UK water sector, where the 1999 periodic review led Ofwat to determine a standard (real post-tax) cost of capital of 4.75%, with minor adjustments for smaller companies. This standard rate was then used to help calculate X.