Summary

The nonfunctional requirements describe the qualitative behavior, or the "how well" qualities, of the productwhether it has to be fast, or safe, or attractive, and so on. These qualities come about because of the functions that the product is required to carry out.

Even something as simple as the common household tap shown in Figure 8.7 has nonfunctional requirements that make the difference between success and failure:

• Look and Feel: The product shall appear easy to operate.

• Usability: The product shall be able to be used by someone with wet hands.

• Performance: The full flow of water shall be achievable with two turns of the handle.

• Operational: The product shall continue to operate correctly with water up to temperatures of 70°C.

• Maintainability: The product shall allow any routine maintenance (such as changing a washer) to be completed in less than 4 minutes by a skilled operator.

• Security: The product shall not be able to be operated by a small child.

• Cultural and Political: The handle shall turn in the direction dictated by local custom.

• Legal: The product shall conform to the Queensland Plumbers and Drainers Board code of installation.

Figure 8.7.

How many nonfunctional requirements does a common household tap have?

At this stage you have written a description and usually a rationale that captures the intention of the nonfunctional requirements. Some of them may seem a little vague, and some of them may appear to be well intentioned and little more. Please keep in mind that you have not finished with these requirements, because you have yet to write the fit criteria. When you do so, you will write a measurement to quantify the meaning of each requirement. We look at this task in the next chapter.