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How Many Design Requirements?

How many requirements should you have? That's a really good question without a good answer. You should have neither too many nor too few. (Big help!)

What is "too many" depends on the product. An airliner might have thousands of design requirements, and that could be just right. For a project that you have time to complete for school, two or three, or maybe five design requirements are appropriate.

The reality is that experience is very important in deciding how many design requirements are important. It's another good time to ask your mentors, parents, and teachers for advice, but do so by asking specific questions. Tell them the design requirements you are considering, and ask them which ones you might be able to do without.

Here are some other thoughts to help you. If you have too many design requirements, it can become very difficult to actually design and build a product. Imagine having a friend whose parents have set ten times as many rules as your parents. Such an imaginary friend might have difficulty doing things because he or she would always be violating one of the rules. Having too many design requirements (requirements are a type of rule) creates a similar situation. With too many requirements, the number of design trade-offs increase, and many design decisions become unnecessarily complex.

Why might too few design requirements be a problem? If you have too few requirements, you might get a result that you don't really want. Let's say that you do not specify a cost requirement. You might end up designing something that costs many times more than what people would be willing to pay for it. Your design would be a failure. So, don't be a slacker on your design requirements.

By the way, professional engineers call a design with too many requirements over constrained (as if it had too many rules) and one with too few requirements under constrained (as if it had too few rules).

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