Saturday, January 12, 2008

Limitations of "The system shall . . . ."

Is your product manager composing documents with hundreds of sentences beginning with "The system shall . . . ." or "The product shall . . . ."? If so, she should consider a different approach.

Back in 2004, Mike Cohn wrote an interesting article on requirements and agile development. Here is an interesting excerpt from the article:
[C]onsider the following requirements:

3.4) The product shall have a gasoline-powered engine.
3.5) The product shall have four wheels.
3.5.1) The product shall have a rubber tire mounted to each wheel.
3.6) The product shall have a steering wheel.
3.7) The product shall have a steel body.

By this point, I suppose images of an automobile are floating around your head. Of course, an automobile satisfies all of the requirements listed above. The one in your head may be a bright red convertible, while I might envision a blue pickup. Presumably the differences between your convertible and my pickup are covered in additional requirements statements.

But suppose that instead of writing an IEEE 830–style requirements specification, the customer told us her goals for the product:

* The product makes it easy and fast for me to mow my lawn.
* I am comfortable while using the product.

By looking at goals, we get a completely different view of the product: the customer really wants a riding lawnmower, not an automobile. These goals are not user stories, but where IEEE 830 documents are a list of requirements, stories describe a user’s goals. By focusing on the user’s goals for the new product, rather than a list of attributes of the new product, we can design a better solution to the user’s needs.
It's revealing that a product satisfying the first alleged "requirements" specification (the series of "The product shall . . . ." statements) likely would fail miserably at addressing the user's real needs. The real requirements are to mow the user's lawn, and for it to be fast, easy, and comfortable for the user.

Via Jonathan Babcock.


Unknown said...


I agree with your observation, but not quite sure if that is the problem of "The System/Product shall......."

What alternative do you have in mind?

Roger L. Cauvin said...

The problem with "The system shall . . . ." is using it to document interaction design.

Requirements are user goals and nonfunctional requirements that we attach to those goals. It is fine to express requirements in terms of "The system shall . . . ."

But when we break those user goals into fleshed out use cases that detail user interactions with the system to achieve the goals, we move into interaction design. Why should we then turn around and write hundreds of "The system shall . . . ." statements to express what is already implicit in the fleshed-out use cases?

Unknown said...

This may be a little far-fetched, but have you tried not to use Use Cases, rather light wieght user stories? Do you really find value in use cases?


Roger L. Cauvin said...

I find tremendous value in both user stories and use cases.