Something old, something new: Requirements and Specifications

For many, perhaps most, development teams the terms requirement and specification are used interchangeably with no detrimental effect. In everyday development conversations the terms are used synonymously, one is as likely to for the “spec ” as the “requirements.” However it is sometimes useful, and occasionally important, to differentiate between the two terms: “A requirement is a desired relationship among phenomena of the environment of a system, to be brought about by the hard-ware/software machine that will be constructed and installed in the environment. A specification describes machine behaviour sufficient to achieve the requirement. A specification is a restricted kind of requirement ” (Jackson and Zave 1995) The key points to note are in the last two sentences: • A specification describes behaviour to achieve requirement. • Specification is a restricted requirement, i.e. the specification narrow