Nevertheless, through our communication, we have sought to take account of the possibility that debt and asset price developments may be a potential source of subsequent instability, but it is neither our ambition nor our conviction that monetary policy can control developments in these variables. Moreover, we have stated in clear terms that the interest rate has been unusually low and that it would have to increase gradually to a more normal level. Since 2005, this has been reflected in our own interest rate forecast. Since inflation expectations have been firmly anchored, we have also been able to increase the interest rate gradually even though actual inflation has been below target. The time horizon for reaching the target in our forecasts has been stretched. Let me now turn to economic models and their use. Even though monetary policy cannot operate with objectives for developments in the labour market, asset prices or debt accumulation, these and other economic variables can provide information about future price developments, which is included in the basis for interest rate setting. In order to glean as much information as possible about future inflation and to gain insight into how the interest rate is affecting the economy, Norges Bank – like other central banks – uses economic models as an analytical tool. What makes a good model depends on the intended purpose of the model. The models' purpose must thus be given decisive weight in the choice and design of modelling tools.
Furthermore, the statistics should be comparable, but not necessarily  2  See Mervyn King, Governor, Bank of England on the occasion of the tenth anniversary of the central bank's independence: (see Financial Times, Thursday, 3 May). 6  BIS Review 102/2007  identical, across countries. It is positive that others are interested in and debate statistics production.3 With an inflation target for monetary policy, measuring inflation is of particular importance. CPI inflation is used as a reference in the wage negotiations each spring. Many financial contracts are linked to CPI inflation. A number of international studies have concluded that there are considerable measurement errors in official consumer price indices.4 The main criticism is that official indices are not sufficiently adjusted for changes in quality and that they do not capture consumers’ possibilities for shifting consumption away from the goods that show the fastest rise in prices. Many of the studies have concluded that consumer price indices overestimate the actual rise in prices. That is one reason why inflation-targeting countries set the target higher than zero. But there are also studies that point out that consumer price indices can underestimate the rate of increase for some components. Against this backdrop, I will now discuss two particular notional concepts used in estimating the consumer price index in Norway to illustrate the uncertainty; how to impute rent for owner-occupied dwellings and how to measure clothing prices.