 The weight (or quantities, to use the above terminology) of an item in the CPI is derived from the expenditure on that item as estimated by the Consumer Expenditure Survey.This survey provides data on the average expenditure on selected items, such as white bread, gasoline and so on, that were purchased by the index population during the survey period.In a fixed-weight index such as CPI-U, the implicit quantity of any item used in calculating the index remains the same from month to month.