The amount of money paid per unit for a good or service. This is easy to observe for many goods and services: in any ordinary shop, customers will find displayed a price at which as many or few units as they wish can be purchased. For some goods and services, however, price is less easy to observe. Special terms may be available for large orders, for repeat orders, or for particular types of customer. In some markets buyers and sellers negotiate over the price of each item. The price of similar goods varies over time and place, and goods with the same name vary in quality. A price index is an aggregate measure of the prices of goods in some specified category: for example, consumer goods, raw materials, or exports. Wholesale prices are the prices charged by wholesalers; factory gate prices are the prices charged by manufacturers. The price mechanism refers to the role of prices in organizing the production and distribution of goods and services in an economy: the prices people are willing to pay convey information about the valuation they put on different goods and services, and the prices charged by suppliers convey information on how they value the effort and inputs needed for production. See also administered price; ceiling price; closing prices; current prices; distorted prices; exercise price; exit price; factor prices; flexible prices; floor price; forward price; law of one price; market prices; mid-market price; opening prices; relative price; shadow prices; shut-down price; spot price; sticky prices.