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and highlighting parts of the sketch. Sketches are possibly easier to comprehend
than language-based scenarios, although images can be susceptible to ambiguous
interpretation. Storyboards and sketches are not testable in themselves but, as with
scenarios, they can provide test cases to validate early requirements with users.
Informal models are less reviewable and comprehensible than sketches since even
informal notations require some learning, but they have the advantage of easier
revision since the format of diagram components enables software tools to facili-
tate editing. Informal models also facilitate testing since their components have an
agreed denotation and dependencies, so associations, links and pathways can be
checked for consistency, although not as rigorously as with formal models. Formal
notations enable comprehensive testing with model checkers and formal reasoning
tools, albeit at the penalty of poor comprehensibility and reviewability for most
stakeholders. Finally, prototypes are comprehensible as the behaviour and appear-
ance is visible; furthermore, user interaction facilitates testing and validation of
requirements. The penalty lies in poor revisability since this necessitates changing
code, although with advanced prototyping languages this cost can be reduced.
Common ground is generated by conversations between stakeholders, augmented
by mutual understanding of information expressed in RE representations. Clearly
the more generally accessible a representation is, the better it supports the formation
of common ground; therefore, not surprisingly, scenarios, storyboards and sketches
are effective means of promoting mutual understanding. However, these representa-
tions are prone to ambiguous interpretation so understanding between stakeholders
may conflict. Informal models can reduce ambiguity by affording easier testing,
while formal models can eliminate ambiguity by automated reasoning, although
with the disadvantage of more difficult comprehension and limited access across
stakeholder groups. An important tenet of Clark's theory is that common ground is
generated not only through conversations but also by interaction with artefacts in
the world. Hence prototypes promote common ground since mutual understanding
is generated by interaction and the consequences of incorrect system actions are
visible to all stakeholders, thereby stimulating validation discussions. Finally it is
worth noting that informal notations in the design rationale family support common
ground by summarising argument and the content of design conversations.
No one representation will suffice to support the development of common
ground; rather, a combination is required to support different activities and phases in
the requirements process when emphasis might change from discovering informa-
tion, to negotiation and establishing common views, then testing and checking that
specifications conform to goals and known facts describing the world. This leads to
the next section, where common ground and RE activities are considered.
4 RE Activities, Representations and Common Ground
The process of collaboration in RE and how this fits within project management
more generally are reviewed using the common ground framework. Lessons from
CSCW for collaboration, such as shared awareness, activity awareness and role
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