Producer Price Indices (PPI) measure average changes in prices received by domestic producers of commodities in all stages of processing. When an establishment is selected to participate in the PPI survey, it is visited by a field economist who solicits the firm's voluntary cooperation and informs the firm of the strict confidentiality rules that will safeguard the information being requested. Once cooperation is obtained, the field economist uses the disaggregation technique to select the goods for which prices will be reported. From this point forward, the establishment reports prices for the selected products, usually on a monthly basis, on a form provided by the BLS. Most information used in calculating the indices is obtained through the systematic sampling of nearly every industry in the manufacturing and mining sectors of the economy. The PPI program also includes some information from other sectors - agriculture, fishing, forestry, services, and gas & electric. Producer Price Indices are designed to measure only the change in prices received for the output of domestic industries, therefore imports are not included. Each month approximately 100,000 prices are collected from 30,000 establishments.