Consumption is a common concept in economics, and gives rise to derived concepts such as consumer debt. Generally, consumption is defined in part by opposition to production. But the precise definition can vary because different schools of economists define production quite differently. According to mainstream economists, only the final purchase of goods and services by individuals constitutes consumption, while other types of expenditure — in particular, fixed investment and government spending — are placed in separate categories. See consumer choice. Other economists define consumption much more broadly, as the aggregate of all economic activity that does not entail the design, production and marketing of goods and services (e.g. "the selection, adoption, use, disposal and recycling of goods and services").[citation needed]
Likewise, consumption can be measured by a variety of different metrics such as energy in energy economics . The total consumer spending in an economy is generally calculated using the consumption function, a metric devised by John Maynard Keynes, which simply expresses consumption as a function of the aggregate disposable income. This metric essentially defines consumption as the part of disposable income that does not go into saving. But disposable income in turn can be defined in a number of ways - e.g. to include borrowed funds or expenditures from savings.

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