The PPI is a key indicator of inflationary trends in the economy. It is used for macroeconomic forecasting.
Since PPI measures the costs of producing consumer goods, and commodity and food prices directly affect retail pricing, PPI is seen as a good pre-indicator of inflationary pressures.
They are calculated quite differently. This directly from the BLS. Just the geometric mean use in CPI makes a big difference.
They are very different measures treated in very different ways. The headline CPI gets much more media attention because CPI
is used for consumer price indexing.....not the PPI.
Other technical differences
Several other technical differences exist between the PPI for personal consumption and the CPI. The PPI and the CPI are both constructed with the use of a modified Laspeyres index formula, but the CPI updates weights annually and the PPI updates weights every 5 years. The CPI also implements a geometric mean formula at the item level that the PPI does not. The geometric calculation reduces substitution bias, leading to lower measures of inflation in periods of price increases. The PPI attempts to collect prices for a specific day of the month (the Tuesday of the week containing the 13th), while the CPI collects prices throughout the month. Finally, prices measured by the CPI include sales and excise taxes, while prices measured by the PPI exclude those taxes.