Prices cover the consumer price index (inflation) and its forecast, the producer price index, the price level index, housing prices and share prices. Consumer price indices measure changes in average retail prices of a fixed basket of goods and services representing household consumption, whereas producer price indices measure average movements of prices received by the producers of various commodities; these are often seen as advanced indicators of price changes throughout the economy. Comparative price levels measure the differences in the general price levels of countries; they are dependent on exchange rates and should be treated with caution. Share price indices are calculated from the prices of common shares of companies traded on national or foreign stock exchanges.