Systems Engineering Principles
Lead Author: Michael Watson, Contributing Authors: Bryan Mesmer, Garry Roedler, David Rousseau, Chuck Keating, Rob Gold, Javier Calvo-Amodio, Cheryl Jones, William D. Miller, Scott Lucero, Aileen Sedmak, Art Pyster
Every discipline has a set of underlying principles which fundamentally characterize it. Discipline knowledge and practice are either explicitly or implicitly derived from its principles. For example, one of the principles of physics is that the speed of light is constant. This specific principle has been validated countless times in practice and is broadly accepted as how the universe works. One common economic principle is that “every choice has an opportunity cost”. http://courses.lumenlearning.com/suny-microelectronics/chapter/reading-the-concept-of-opportunity-cost/ Another is the principle of supply, which states that “the quantity of a good supplied rises as the market price rises and falls as the price falls”. (https://www.econlib.org/library/Enc/Supply.html). Generally, disciplines do not have one authoritative complete set of principles. Different authors emphasize different aspects of a discipline in the principles they identify and explain. As fields mature, new principles emerge, and old principles may change. For example, Einstein’s Theory of Relativity revealed that the principles underlying Newtonian physics were not absolutely true. Yet, they are still immensely valuable most of the time.
Systems engineering exists to develop a solution to meet a need. This is the motivation of systems engineers in accomplishing their work. But how do system engineers accomplish this expansive challenge? To address this, a set of systems engineering processes exist and recently a set of systems engineering principles to articulate the basic concepts that guide systems engineering processes have emerged. The INCOSE Systems Engineering Principles Action Team (SEPAT) reviewed various sources of systems principles and systems engineering principles identified in literature. This article surveys this work and identifies a set of systems engineering principles based on this review.
System principles and systems engineering principles differ in important ways (Watson, 2018a). System principles address the behavior and properties of all kinds of systems, looking at the scientific basis for a system and characterizing this basis in a system context via specialized instances of a general set of system principles. SE principles are a specialized and contextualized instantiation of systems principles that address the approach to the realization, use, and retirement of systems. SE principles build on systems principles that are general for all kinds of systems (Rousseau, 2018a) and for all kinds of human activity systems (Senge, 1990, Calvo-Amodio & Rousseau, 2019). Hence, system principles guide the definition and application of SE processes – a strong systems engineer must be the master of both system principles and SE principles.
SE is a young discipline, but a number of people have proposed its underlying principles, in part by building on system principles. Perhaps because of its youth, there is no consensus among the community about which SE principles are most central or even which ones are valid. Yet, it is valuable to examine a number of proposed SE principles as they offer insight into what some of the best minds in the field think are fundamental to the discipline. In reviewing various published SE principles, a set of criteria emerged for valid SE principles. A principle:
- transcends a particular lifecycle model or phase
- transcends system types
- transcends a system context
- informs a world view on SE
- is not a “how to” statement
- is supported by literature or widely accepted by the community; i.e. has proven successful in practice across multiple organizations and multiple system types
- is focused, concise, and clearly worded.
Thus, system type, the context in which the system is developed and operated, or a life cycle phase do not narrowly define systems engineering principles. Systems engineering principles transcend these system characteristics and inform a worldview of the discipline. Principles are not “how to” statements, which are embodied in the processes, but provide guidance in making decisions in applying the systems engineering processes. Principles should have a strong reference basis supported by literature, or widely accepted in practice (keeping in mind that this success must transcend the system characteristics mentioned above), or both. Principles are focused, concise, and clear in well-constructed principles statements.
Literature currently contains several good articles on system principles. These principles provide a basis for the functioning of a system and seek to group scientific axioms, laws, and principles into a set of system principles. The main themes seen in the literature on system principles include system governance, system theory axioms, and system pathologies with a focus on complex systems and system of systems. Complex System Governance provides a set of nine Metasystem functions “to provide control, communication, coordination, and integration of a complex system”. These functions provide a basis from which to understand the functions of complex systems and how to manage their acquisition (i.e., governance). (Keating, et al, 2017a) These Metasystem functions also extend to systems of systems engineering. (Keating, et al, 2017b)
Advances in system theory produced a set of unified propositions stated as seven axioms “from which all other propositions in systems theory may be induced”. These seven axioms map to 30 scientific laws and principles (Adams, et al, 2014). These axioms focus on the scientific basis of systems. Further work on these axioms provides an integration construct and a slightly different mapping to the underlying scientific laws and principles (Whitney, et al, 2015). This work provides a strong integration and advancement in system theory, focusing on the principles behind the scientific basis of a system.
System science approaches also incorporate system theory leading to 10 concepts of systems theory and systems thinking (Sillitto, 2014). These 10 concepts focus on system principles providing a definition of system characteristics. A further development in system sciences produced a list of 12 systems sciences principles that also focus on the characteristics of systems (Mobus & Kalton, 2015). Rousseau formally derived a statement and derivation of three principles of systems (Rousseau, 2018a). In addition, an architecture of systemology and typology of system principles provides a good classification of scientific principles spanning from system philosophy through system practice (Rousseau, 2018b). This work led to a framework for understanding system science principles (Rousseau, 2018c).
Other early work included a set of seven system science principles exhibited by systems (Hitchins, 1992). Organizational principles where also defined as a set of 11 principles dealing with how to work successfully within an organization (Senge, 1990). Principles of Systems Thinking describes a set of 20 system thinking principles captured and integrated from a variety of sources.
System pathologies is another interesting approach to understand “circumstances that act to limit system performance or lessen system viability (continued existence) and as such they reduce the likelihood of a system meeting performance expectations”. These pathologies define diagnostics for understanding systems derived from a set of 45 system laws and principles (Katina, et al, 2016).
INCOSE compiled an early list of principles. These principles consisted of 8 principles and 61 sub-principles (Defoe, 1993). These principles were important considerations in practice for the success of system developments and ultimately became the bases for SE processes. These principles are reflective of how SE works in general. Following this work, several early versions of SE principles were compiled leading up to one of the first documented sets of SE processes. Project Performance International (Halligan, 2019) has a set of SE principles that follow along the model set by Defoe providing considerations in the practice of SE, focusing on specific aspects within life cycle phases.
The Korean Council on Systems Engineering provided a survey article of 8 works on SE principles spanning the time from Defoe’s principles through 2004 (Han, 2004), including an early version of the PPI principles. These 8 works showed evolution of systems engineering principles from practice focused to more transcendent focused principles. In 1997, the INCOSE Systems Engineering Principles Working Group (no longer active) generated a set of 8 principles building from the work of Defoe over the course of several years of discussions. These principles were a mixture of process basis, modeling guidelines, and an early world view of the SE focus. The Institute of Electrical Engineers (IEE, 2000), now part of the Institution of Engineering and Technology (IET), produced a set of 12 principles that also provided some basis for the systems engineering processes which are no longer extant. Lawrence Berkley National Laboratory (LNBL, 2001) produced a set of systems engineering principles that embody the concepts captured by the INCOSE SE processes. In England, the Defence Engineering Group (DEG, 2002) produced an SE Handbook with a brief set of principles guiding their processes and capturing some aspects of systems principles. Iowa State University is reported to have produced an SE Student Handbook containing a short list of SE heuristic phrases stated as principles. The KCOSE paper also referenced a lecture on SE principles from a course at the University of Southern California (USC) (Jackson, 2003). This lecture defined a principle as “a statement or generalization of a truth reflected in the systems engineering process”, showing the focus on processes in the early SE principle development.
Some early forms of SE principles were also contained in text books on complex system development (Adamsen II, 2000). This set of principles assume a hierarchical system representations (complex systems have since shown to be more networks than hierarchies) and include statements on SE processes. Finally, system architecting books also included some early SE heuristics (Maier and Rechtin, 2002). These heuristics read as sayings about some aspect of systems engineering practice.
The KCOSE Technical Board reviewed these 8 sources and voted that 8 of the principles from these sources as a set of SE principles, leading to an early form of transcendent principles consistent with the criteria defined above. These sources all show the early evolution stages of the SE principles as people looked at both formal and informal (i.e., course notes and student handbooks) sources to try and understand SE principles. The definition of the SE processes in works such as the INCOSE Systems Engineering Handbook fulfilled some of the objectives of these early works on SE principles and consolidated a lot of the work in this area. Recently, the need for more transcendent SE principles has been recognized, as a guide for applying the processes, which is the focus of current work by the SEPAT.
Over the last several years, a fresh look at the set of SE principles has emerged from the SEPAT (Watson, et al., 2019). Its effort is based on SE postulates, principles, and hypotheses from the NASA Systems Engineering Research Consortium. This consortium followed the approach of Ludwig Boltzmann in defining his postulates on gas distribution laws. Boltzmann’s work is an early example of how to characterize the interactions of complex systems. A postulate is something assumed without proof to be true, real, or necessary (Webster 1988). This led to the articulation of a set of postulates and hypotheses underlying SE which were expanded into a proposed set of SE principles. The underlying SE postulates and hypotheses matured over the course of 4 years (Watson, et al, 2014; Watson, et al, 2015; Watson & Farrington, 2016). As the postulates matured so did the SE principles, providing more specifics in the application of SE, and the proof of a hypothesis becoming a principle (Watson, et al, 2018; Watson, 2018b). The SEPAT developed 15 principles, some expanded by subprinciples, described in (Watson, et al, 2019). These Principles are:
- SE in application is specific to stakeholder needs, solution space, resulting system solution(s), and context throughout the system life cycle.
- SE has a holistic system view that includes the system elements and the interactions amongst themselves, the enabling systems, and the system environment.
- SE influences and is influenced by internal and external resource, political, economic, social, technological, environmental, and legal factors.
- Both policy and law must be properly understood to not overly constrain or under constrain the system implementation.
- The real physical system is the perfect model of the system.
- A focus of SE is a progressively deeper understanding of the interactions, sensitivities, and behaviors of the system, stakeholder needs, and its operational environment.
- Stakeholder needs can change and must be accounted for over the system life cycle.
- SE addresses stakeholder needs, taking into consideration budget, schedule, and technical needs, along with other expectations and constraints.
- SE decisions are made under uncertainty accounting for risk.
- Decision quality depends on knowledge of the system, enabling system(s), and interoperating system(s) present in the decision-making process.
- SE spans the entire system life cycle.
- Complex systems are engineered by complex organizations.
- SE integrates engineering disciplines in an effective manner.
- SE is responsible for managing the discipline interactions within the organization.
- SE is based on a middle range set of theories.
The SEPAT’s recent articulation of SE principles elaborates on points made earlier by Defoe and emphasizes additional aspects of current SE practices, but there is nothing inconsistent between the two sets. Principles of SE such as those proposed by Defoe and the SEPAT are domain independent; i.e. they apply independent of the type of system being built, whether it is for transportation, healthcare, communication, finance, or any other business or technical domain. As they are applied, these principles can take more specialized forms, and/or can be complemented by other context-specific principles. Indeed, general SE principles such as these have been successfully applied in virtually every domain.
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