Requirements and specifications: What's the difference and what's it to you?

There have been a number of threads I have followed in a few different forums recently where people have discussed requirements, what it means for requirements to be ‘good’, and what it might mean for requirements to be unambiguous. What usually follows is a long-winded back and forth, with no resolution.

At the heart of this deja vu is the fact that one person’s requirements are not necessarily another’s. I resolve this by drawing a clear distinction between specifications and requirements. The distinction may be obvious to some, but in practice it seems to be something we struggle with.

In “Software for dependable systems: Sufficient evidence?”, Daniel Jackson and his co-authors take care to highlight this issue:

“Software systems that are developed specially for a particular client are typically built to meet preagreed requirements, but these requirements are often a long and undifferentiated list of detailed functions.”

“The requirements of a software system should describe the intended effect of the system on its environment and not, more narrowly, the behavior of the system at its interface with the environment, which is the subject of the specification.”

They also take care to point out that the environment of the system includes the software product, plus the humans that use them, and other environmental factors external to the software.

The specifications are not about things that anybody *needs*. Specifications represent the end result of negotiations, conversations, politics and expediency that some group of people thinks represents an understanding that is good enough for now. The specification is at best, our best guess of what’s going to make the world a better place for the numerous people who have a stake in the thing that we’re building. Specifications are a waypoint on the path to something else.

Specifications are never equivalent to requirements in the case of things that will be used by humans.

Specifications apply to the pointy end of a screwdriver that needs to fit into the indented part of some screw.

As testers, testing to specifications is something we do because finding about the requirements is too hard. We do it because that’s what the testers before us did. We do it because the process might be built around specifications documents, and that’s what managers are tracking to. We might reasonably test to spec if our job is just to test a software component that’s on its way to be integrated with something else. However, testing to spec can’t tell us that the system is going to yield the desired benefits.

In the case of the screwdriver, requirements apply to the handle – how it fits your hand, and the hands of others. They apply to whether it gives you enough grip and whether it has a switch to make it only screw or unscrew. They apply to whether it fits enough hands in the world, and whether it can be built to a price that someone is willing to pay. Requirements apply to whether the pointy end of the screwdriver will make do for unscrewing (or screwing in) a screw that is not of the size covered by the specification for the pointy end of our screwdriver.

Requirements are about utility and real human problems, and are fuzzy, and messy, and never fully understood. When our stakeholders ask us questions about testing, though they often don’t phrase it this way, they are usually interested in information with respect to requirements, both explicit and implicit.

Test to see how the product measures up to requirements, and to learn about what the requirements are. That’s the value that you can bring to the project.