By way of the definition of a price-effect and of a consumption-effect, an attempt is made to demonstrate up to what point is it technically incorrect to take, as equivalents or as similar, the time evolutions of the structures of spending and of consumption, when changes are taking place in relative prices. By virtue of the interest which the application of this analysis has to the case of the weightings used in the calculation of price indexes, an analysis is carried out of the successive changes to the base of the Cost of Living Index and of its successor, the Consumer Price Index, since 1936.