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are named as sectors. They are very different from each other. They are also called as the primary, secondary, and tertiary sector. They are:
• The extraction of raw materials (Primary sector)
• Manufacturing goods (Secondary sector)
• Providing services (Tertiary sector)
Petty’s Law reveals the evolvement rule of labor force’s distribution structure in the three industrial sectors during the process of economic development. Because of the relative income differences among different sectors, labors tend to move to sectors with higher income levels.
2.42 Assumptions
There are three significant assumptions of Petty’s Law.
• This law studies the evolution rule of industrial structure, which is based on the changes that occurred in several countries over time. This time series corresponds to the rising national average income level.
• This law studies the evolution rule of industrial structure, in which labor force is used as an indictator. It studies the changes of distribution of labor force in different sectors during the process of economic development.
• The basic framework of this law is the three sector classification method. All economic activities are classified into primary sector, secondary sector, and tertiary sector. Petty’s Law summaries the experience of changes of industrial structure. It not only can be proved from the time series of economic development of one country but also can draw similar conclusions from the comparison of various countries at different development levels at a fixed time point. That is to say, in terms of economic conditions of various countries at different development levels at a fixed time point, in countries where the national average income level is relatively low, the proportion of labor force in the primary sector is relatively large, while the proportion of labor force in the secondary and tertiary sectors are smaller. On the contrary, in countries where the national average income level is relatively high, the proportion of labor force in the primary sector is relatively low, while the proportion of labor force in the secondary and tertiary sectors are larger. Therefore, Petty’s Law is an economic law reflecting the industrial structure changes.
2.43 Main Content
The main content of Petty’s Law is: due to the income differences in different industries, labor forces tend to move to sectors with higher income levels. With the improvement of national average income level, labor forces would move from primary sector to secondary sector. With the further increase of national average income level, labor forces would move to the tertiary sector. As a result, in terms of distribution of labor force in various sectors, the proportion in primary sector will decrease, while the proportion in secondary and tertiary sectors will increase.

3. Research purpose, question and plan研究目的、问题和计划

3.1 Research purpose

Industrial structure issue of cities is the core theoretical problem of studying urban economy. Reasonable industrial structure is the prerequisite for the healthy development of the urban economy. Industrial structure issue not only is advantageous to fully explore the urban resources, give full play to city’s advantages, improve the economic performances of urban sectors, str