System boundary chart

Now that you’ve had a look at the how your users interact with your problem, it’s worth defining your scope. Constructing a system boundary chart is the first step in defining what’s in and out of your system. The system boundary chart is flexible and changeable, but it explicitly defines how you consider the components, areas or influences of your system.

Example applications

This tool can be used as a brainstorming activity for many things. Typical uses include helping you to define your scope, or deciding what components are inside or outside your concern in a subsystem interface diagram.

Steps

1. Brainstorm all of the variables you can think of about the problem. Think widely, and add in everything you can think of.
2. 10.Categorise all of the variables into one of the three aspects:
   • • Internal (inside) - these are variables that are part of your system of interest, and you have direct (or mostly direct) control over them (for example, the components of the system)
     • External (outside) - these are variables that interact with your system of interest, but you do not have direct control over them (for example, the user interacting with the system)

• Excluded (exogenous) - these are variables that might be important to consider at some stage, but for the time being are considered outside of the scope (for example, the weather)

Hints

• It’s important to make sure that you revisit problem scoping throughout the design process to make sure that you haven’t scoped the problem incorrectly
• The excluded list could be everything-in-the-world-that’s-not-inside the system. Be judicious but comprehensive with your list.

Core resources

• Sterman, J.D., 2000, ‘Business Dynamics - Systems Thinking and Modelling for a Complex World’, McGraw Hill The model boundary chart (p97-98) [PDF, 6 pages]
• Define the System Boundaries A Practical Guide to Security Engineering and Information Assurance [PDF, note read especially p67-72]