Systems Engineering

Input-Output Models were first developed for quantitative understanding of the total flows in an economy. They can be applied to any system, not just economic ones, for determining if all the inputs and outputs of a system add up. It can be visualized as a spreadsheet with the elements of a system as rows, and additional rows for items outside the system. Types of inputs and outputs are in columns. Each component requires inputs such as power, data, fuel, etc. It also produces some kind of output. The purpose of a model is to see if your system as a whole has closure and balance. In other words, are all the inputs matched by outputs? Are there missing components identified by missing inputs? Is the size or quantity of a particular element in the system the correct size/productivity? Will the system as a whole produce the desired output, and if so how much? These are really all questions of accounting. Rather than counting everything in money, this type of spreadsheet does the accounting of each type of input/output/resource/supply separately as categories. Note that human labor is one of the input types.

A model, such as an Input/Output Model lets you actively see how a change in any one component, such as a new design, impacts the system as a whole. By summing the flows of the component functions in a model spreadsheet (or other computer model) you can immediately find changes to the rest of the system components and the totals for the entire system. The Input/Output model and functional diagrams both model aspects of the same system, and may be combined within a single software tool or database if it can represent all the details of a system in sufficient detail. This is desirable for plotting how changes in system components affect the system as a whole.

Functional diagrams at a basic level are maintained as static drawings, and input/output models can be an actual spreadsheet. Using the same numbering system and structure for the diagrams and models maintains the relationship between them. They are both representing the same system, just different aspects.