Three-sector model
The three-sector model in economics divides economies into three sectors of activity: extraction of raw materials (primary), manufacturing (secondary), and service industries which exist to facilitate the transport, distribution and sale of goods produced in the secondary sector (tertiary). The model was developed by Allan Fisher, Colin Clark, and Jean Fourastié. in the first half of the 20th century, and is a representation of an industrial economy. It has been criticised as inappropriate as a representation of the economy in the 21st century.

According to the three-sector model, the main focus of an economy's activity shifts from the primary, through the secondary and finally to the tertiary sector. Countries with a low per capita income are in an early state of development; the main part of their national income is achieved through production in the primary sector. Countries in a more advanced state of development, with a medium national income, generate their income mostly in the secondary sector. In highly developed countries with a high income, the tertiary sector dominates the total output of the economy.

The rise of the post-industrial economy in which an increasing proportion of economic activity is not directly related to physical goods has led some economists to expand the model by adding a fourth quaternary or fifth quinary sectors, while others have ceased to use the model.
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