A measurement of purchasing power would have to rely upon the prices of the goods and services of the first order and, what is more, of all of them. To employ the prices of the producers' goods is not helpful because it could not avoid counting the various stages of the production of one and the same consumers' good several times and thus falsifying the result. A restriction to a group of selected goods would be quite arbitrary and therefore vicious. But even apart from all these insurmountable obstacles the task would remain insoluble. For not only do the technological features of commodities change and new kinds of goods appear while many old ones disappear.
Valuations change too, and they cause changes in demand and production.
The assumptions of the measurement doctrine would require men whose wants and valuations are rigid. Only if people were to value the same things always in the same way, could we consider price changes as expressive of changes in the power of money to buy things. As it is impossible to establish the total amount of money spent at a given fraction of time for consumers' goods, statisticians must rely upon the prices paid for individual commodities. This raises two further problems for which there is no apodictic solution. It becomes necessary to attach to the various commodities coefficients of importance.