There are a large number of definitions of a system. We will define a system as an interacting group of items forming a unified whole. According to O’Connor and McDermott, the key to identifying a system is that a system maintains its existence and functions as a whole through the interaction of its parts. They write:
Your body is the perfect example. Your body consists of many different parts and organs, each acting separately, yet all working together and each affecting the others. Your thoughts affect your digestion and heart beat, the state of your digestion affects your thoughts—especially after a large lunch. The eye cannot see, nor the legs move without a blood supply, and the blood supply has to be oxygenated through the lungs. The movement of the legs helps pump the blood back to the heart. The body is a complex system.6
It is important to appreciate the features of a system and the implications of a system. O’Connor and McDermott distinguish a system from what they call a heap or collection of pieces as follows:
• A system is a series of interconnected parts which function as a whole. A heap is merely a collection of parts.
• A system changes if you take away or add pieces; if you cut a system in half, you do not get two smaller systems: you get a system that will not function. A heap can be divided into pieces, each of which can function on their own.
• In a system, the arrangement of the pieces is crucial, while in a heap, the arrangement is irrelevant.
• In a system, the parts are connected to each other and work together, while in a heap, the arrangement of the pieces is irrelevant.
• The behavior of a system depends on its overall structure, while in a heap, size rather than structure determines behavior.
As we have seen, the term “system” may be used to describe complex biological relationships. “System” may also be used to describe organizations’ relationships or processes, such as a healthcare system, a public health system, or a research system. Alternatively, “system” may be used to describe the working of factors or influences that bring about disease and the outcome of disease. Each of these uses of the term “system” shares the intention of understanding how the pieces or items fit together in a coherent whole. In recent years, population health has increasingly turned to systems thinking to better understand the operation of organizations and processes, as well as the development and outcome of disease.c
Implementing or operationalizing systems thinking requires tools for analyzing the pieces and understanding how they fit together. This process is called systems analysis. There are a wide range of these tools. Often, systems analysis relies on diagrams or graphics that visually display the relationships between the parts and allow us to better understand how the parts fit together and work together.