<aside> đźš© The first step in systems design is to define and quantify all the relevant input and output flows of mass, energy, and information.
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<aside> <img src="/icons/info-alternate_blue.svg" alt="/icons/info-alternate_blue.svg" width="40px" /> Read about the basic definition and characteristics of Systems and System Interfaces before studying this page.
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Interventions are Systems that alter situations (environments) by changing things given to them (inputs) to things needed by users and other systems (outputs).
Without knowing exactly how much of each input you have to work with, and exactly how much of each output is expected or acceptable, you cannot create a suitable intervention. Thus, when designing such a system, the first step is to define and quantify the relevant inputs and outputs.
The inputs will come from the environment, over which you have no control whatsoever. You will have to quantify those inputs and design an intervention that can “handle” those inputs in an effective, efficient, usable, and safe manner.
The outputs are largely derived from your Requirements, which means they are defined by the expectations and demands of your users, and other elements of the environment (e.g., regulations, the laws of nature, other systems with which your intervention will have to interact, etc.).
When designing an intervention as a system, the inputs and outputs are defined via interfaces between the intervention being designed and other systems in the environment.
The other systems in the environment are not under your control; you must accommodate for them and can use them in your design, but you cannot change them.
The outer-most inputs and outputs of your intervention, that interact directly with the environment, are specified with System Interfaces. What distinguishes these outer-most inputs and outputs is that you do not control them.
For instance, you do not just decide how much current an electric motor will draw; you must instead determine how much current can be provided by the environment and configure an electric motor that will use no more than the available current. Similarly, you cannot arbitrarily decide how much ink the pen that you are designing will hold; you must determine this based on how long your users want to write between refilling tasks.
These overall interfaces make no commitment to a particular type of intervention (e.g., electric powered versus manually powered) that would exclude a type of intervention without clear and robust evidence (e.g., a direct requirement arising from the Design Brief).
Therefore, the list of overall interfaces will apply to all Design Concepts. Thus, one such list is all that’s needed for each team.
To have a complete design, you must specify interfaces for all the inputs that your environment may reasonably produce as flows into your intervention.
Not all the input flows will be “good” (i.e., beneficial) flows; some of them will be “bad” (i.e., detrimental) flows.
A good input is one that your intervention needs or can use to produce the required outputs. A bad input is one that will hinder or prevent your intervention from producing the required outputs.
Bad inputs are “real life”; there are always bad inputs, and you cannot prevent that. A diligent designer will manage those bad inputs so that the intervention can still perform well. Managing bad inputs is a key task for every designer.