The CPI and PPI differ chiefly in whose prices they track, with the PPI tracking prices paid by:

1. Each index has a defined population of buyers. 2. The most commonly cited measure of inflation is the Consumer Price Index. 3. The Producer Price Index is a separate measure. 4. It is based on prices paid for supplies and inputs. 5. Those are paid by producers of goods and services. _Source: OpenStax Principles of Macroeconomics for AP Courses 2e (CC BY 4.0), Ch 8 'Inflation'_