Hierarchical classification is a system of grouping things according to a hierarchy.
In the field of machine learning, hierarchical classification is sometimes referred to as instance space decomposition, which splits a complete multi-class problem into a set of smaller classification problems.
The Chinese city tier system (Chinese: 中国城市等级制; pinyin: Zhōngguó chéngshì děngjízhì; lit. 'Chinese city hierarchy') is an unofficial hierarchical classification of Chinese cities in the People's Republic of China (PRC). There are no such official lists in the country, as the Chinese government does not publish or recognize any official definition or a list of cities included in the tier system. However, it is frequently referred to by various international media publications for purposes including commerce, transportation, tourism and education, among others.
Given the rapid development of Chinese cities ever since the economic reform and the ever-changing dynamics among the country's cities, the tier system has gained wide popularity in recent years as a point of reference, such as among investors and tourists. Cities in different tiers often reflect differences in consumer behavior, income level, population size, consumer sophistication, infrastructure, talent pool, and business opportunity, among others. The tier system typically includes cities in mainland China only. The special administrative regions (SARs) of Hong Kong and Macau are not included on the list.
Chemistry is often called the central science because of its role in connecting the physical sciences, which include chemistry, with the life sciences, pharmaceutical sciences and applied sciences such as medicine and engineering. The nature of this relationship is one of the main topics in the philosophy of chemistry and in scientometrics. The phrase was popularized by its use in a textbook by Theodore L. Brown and H. Eugene LeMay, titled Chemistry: The Central Science, which was first published in 1977, with a fifteenth edition published in 2021.
The central role of chemistry can be seen in the systematic and hierarchical classification of the sciences by Auguste Comte. Each discipline provides a more general framework for the area it precedes (mathematics → astronomy → physics → chemistry → biology → social sciences). Balaban and Klein have more recently proposed a diagram showing the partial ordering of sciences in which chemistry may be argued is "the central science" since it provides a significant degree of branching. In forming these connections the lower field cannot be fully reduced to the higher ones. It is recognized that the lower fields possess emergent ideas and concepts that do not exist in the higher fields of science.