PCs describe what a product ought to be, but not what the product ought to do. As such, PCs can almost always be described using adjectives and adjectival phrases. A PC is never phrased as a verb (that what a functional requirement is for).

At first glance, PCs might seem almost vacuous, contributing nearly nothing to the study of the design problem. However, this is not really the case. A common source of error in design is that the designers will forget to consider the impact of one or more PCs in their work. It is human nature: designers will tend to focus on issues that are of particular importance to them, and expect someone else to take care of other issues. Sometimes, some issues will “fall through the cracks” resulting in a poor design¹⁾.

If you think about it, there are some PCs that every designed intervention has. Because of this we can define a fundamental list of PCs that everything you design will be. The real questions are how and to what extent will your interventions exhibit these characteristics.

To simplify matters in this course, all teams must use only the following five product characteristics.

Functionality

Does the design intervention actually do what it's supposed to do? Does it fulfil its intended role in a larger system? Does it bring about the expected/preferred situation?

Usability

Can users use it? Can they use it safely, easily, even enjoyably?

Producibility

Can the intervention be manufactured/assembled easily and robustly?

Maintainability

Can the intervention be maintained easily and well? Does it even need to be maintained? How inconvenient is maintenance to users?

Sustainability

What impact does the design impose on the environment? How long will the product last? What happens to it at its end of life? How many people can afford it and for how long?

None of the questions above are binary - there are no “yes-or-no” answers. The answers to all these questions exist on a spectrum: any design can be ranked on how much they are better or worse with respect to each of these five PCs.

There are many possible adjectives that could be PCs, but are not included in this list.

• For instance, durability, lightweightedness, robustness, and safety, are all adjectives that could be used to capture important aspects of a design.
• There are two significant reasons for not using these other adjectives.
  1. They partly overlap in meaning with one or more of the five PCs above. Overlapping PCs confuse matters and increase the odds of eventually creating conflicting requirements.
  2. They can all be represented by constraints on the five PCs or on various FRs. For instance:
     • durability can be captured with constraints on wear, product longevity, time between maintenance, etc.,
     • lightweightedness can be captured with constraints on weight,
     • robustness can be captured with constraints on the behaviours a product has when its operating inputs vary near or beyond its design limits, and
     • safety can be captured with constraints on failure modes and effects, protection of users, avoidance of accidents, etc.