Toward a property based requirements theory: System requirements structured as a semilattice

In this paper, we outline a property‐based requirement (PBR) theory. After introducing the mathematical notion of semilattice, we express the ontological and epistemological assumptions on which our theory is built. We provide a definition of a “well formed (wf‐)requirement,” based on the property concept, distinguished from that of an “expectation.” Then, we introduce two relationships between wf‐requirements called respectively “conjunction” operation and “stringency” relationship. So, we state that a set of wf‐requirements, assigned to a system, is structured as a semilattice and that the maximum element of this semilattice matches the system specification. The network including all the expectations and wf‐requirements related to a system are linked together through two very unlike categories of relationships. “De dicto” relationships are linguistic dependencies while “de re” relationships among requirements are extralinguistic dependencies among material properties refereed by wf‐requirements. We highlight two types of “de re” relationship among wf‐requirements. The first one is the well known “derivation” relationship that links together a “parent” requirement with its “child” requirements during the system design process. We introduce a second one, called “coupling” relationship. Wf‐requirements are coupled when the change of one of them collides with others. We claim that our PBR theory belongs to the same paradigm as the model‐based systems engineering (MBSE) approaches whereas TBR belongs to the same field as empirical design approaches. Then, we relate our requirements formulation to features of the OMG SysMLTM language. We conclude on our prospects for research, which connect our requirement theory with reflections carried out on the bodies of knowledge and their specification, i.e. ontologies. © 2008 Wiley Periodicals, Inc. Syst Eng