Even if index numbers cannot fulfill the demands that theory has to make, they can still, in spite of their fundamental shortcomings and the inexactness of the methods by which they are actually determined, perform useful workaday services for the politician. If we have no other aim in view than the comparison of points of time that lie close to one another, then the errors that are involved in every method of calculating numbers may be so far ignored as to allow us to draw certain rough conclusions from them. Thus, for example, it becomes possible to a certain extent to span the temporal gap that lies, in a period of variation in the value of money, between movements of Stock Exchange rates and movements of the purchasing power that is expressed in the prices of commodities.

The Theory of Money and Credit