How to combine the relative changes in the prices and quantities of various products into (i) a single measure of the relative change of the overall price level and (ii) a single measure of the relative change of the overall quantity level. Or, conversely, how a value ratio pertaining to two periods of time can be decomposed in a component that measures the overall change in prices between the two periods - that is the price index - and a component that measures the overall change in quantities between the two periods - that is the quantity index.