Grey relational analysis, also called Deng's Grey Incidence Analysis model, was developed by a Chinese Professor Julong Deng of Huazhong University of Science and Technology. It is one of the most widely used models of Grey system theory. GRA uses a specific concept of information. It defines situations with no information as black, and those with perfect information as white. However, neither of these idealized situations ever occurs in real world problems. In fact, situations between these extremes, which contain Dispersed knowledge, are described as being grey, hazy or fuzzy. A variant of GRA model, Taguchi-based GRA model, is very popular in engineering.
GRA is an important part of grey system theory pioneered by Professor Deng in 1982. A grey system means that a system in which part of information is known and part of information is unknown. With this definition, information quantity and quality form a continuum from a total lack of information to complete information – from black through grey to white. Since uncertainty always exists, one is always somewhere in the middle, somewhere between the extremes, somewhere in the grey area. Grey analysis then comes to a clear set of statements about system solutions. At one extreme, no solution can be defined for a system with no information. At the other extreme, a system with perfect information has a unique solution. In the middle, grey systems will give a variety of available solutions. Grey analysis does not attempt to find the best solution, but does provide techniques for determining a good solution, an appropriate solution for real world problems. The theory inspired many noted scholars and business leaders like Jeffrey Yi-Lin Forrest, Sifeng Liu, Ren Zhengfei and Joseph L. Badaracco, a professor at Harvard Business School. Dr. Sifeng Liu, the pupil of Deng, building upon the work of Deng's GRA model proposed his own Absolute GRA model. In 2018, Liu and his doctoral student merged Deng's GRA model and Absolute GRA model to propose the Second Synthetic GRA model. The theory has been applied in various field of engineering and management. Initially, the grey method was adapted to effectively study air pollution and subsequently used to investigate the nonlinear multiple-dimensional model of the socio-economic activities’ impact on the city air pollution. It has also been used to study the research output and growth of countries. In the world there are many universities, associations and societies promoting Grey System Theory e.g., International Association of Grey Systems and Decision Sciences, Chinese Grey System Association, Grey Systems Society of China, Grey Systems Society of Pakistan, Polish Scientific Society of Grey Systems, Grey Systems Committee, Centre for Computational Intelligence etc. There are several journals dedicated to grey systems research and studies e.g., "The Journal of Grey System", "Grey Systems Theory and Application", "Journal of Grey System", "Grey Room", "The Grey Journal", "Grey Matters", Journal of Intelligent and Fuzzy Systems, Kybernetes, etc.
