in prices received for a product or service regardless of industry of origin. It organizes products by similarity, end use, or material composition. This system is unique to the PPI and does not match any other standard coding structure, such as the SIC or the U.N. Standard International Trade Classification (SITC). Historical continuity of index series, the needs of index users, and a variety of ad hoc factors were important in developing the PPI commodity classification. Final Demand-Intermediate Demand (FD-ID) Final Demand-Intermediate Demand (FD-ID) indexes are made up of PPIs in the commodity system. This system regroups commodity indexes to create aggregate PPIs according to the type of buyer and the amount of physical processing or assembly products have undergone. The FD-ID system replaced the PPI "stage-of-processing" (SOP) system as PPI's primary aggregation model with the release of data for January 2014. The scope of the SOP system was narrower than the PPI index. Over 600 FD-ID PPIs are available measuring price change for goods, services, and construction sold to final demand and intermediate demand. The final demand portion of the FD-ID system measures price change for commodities sold as personal consumption, capital investment, government purchases, and exports. The intermediate demand portion of the FD-ID system tracks price change for goods, services, and construction products sold to businesses as inputs to production, excluding capital investment. Calculating Index Changes Movements of price indexes from one month to another are expressed as percent changes, rather than as changes in index points. Each index measures price changes from a reference period defined to equal 100.0. Currently, some PPIs have an index base set at 1982 = 100, while the remainder have an index base that corresponds with the month prior to the month that the index was introduced. BLS measures price change in relation to that figure. An index level of 110, for example, means there has been a 10% increase in prices since the base period; similarly, an index level of 90 indicates a 10% decrease in prices. To calculate the percent change in prices between some previous period and a more current period using a PPI, the BLS uses the following formula: Current period index level - Previous period index level = Index point change Index point change ÷ Previous period index level = Proportion of change Proportion of change × 100 = Percent change For example, in the first quarter