Information System Development Life Cycle Models - High Level Models and Methodologies for Information Systems - page 25

There is a variation of the incremental model, named the iterative and incre-
mental development model (IIDM). While very similar to incremental, this model
puts greater emphasis on the relationships that occur with each increment and
between them. These relationships, or iterations, form a cycle or pattern of feed-
backs and outputs. In that regard, while maintaining the essence of the incremental
model, the IIDM is a more fluid description of development. At the same time, it

also allows more space and signi cance for feedback, as it modi es the scheduling

strategy to include speci c time frames for revision and improvement of each

increment, so that successful conclusion of the development process is more likely
at the rst final version (Cockburn 2008 ).

2.4 The Spiral Life Cycle Model

The spiral model dates back to the end of the 1980s, when it was outlined by Barry
Boehm, and introduces something that other models did not take into account,
which is risk analysis. In essence, the spiral model attempts to bring together key
aspects of some other prominent models (namely the waterfall, incremental, and
evolutionary prototyping), in an attempt to gather the most appropriate traits from
each one, because specific projects might be more or less adaptable to specific
models.
According to this SDLC model, the process of developing a system consists of a
series of cycles or iterations. Each cycle begins with the identi cation of objectives

and requirements of the current stage, as well as an analysis of alternatives and
constraints. This process will highlight areas of uncertainty (risk), which will be
taken into account during the next step, the outlining of a strategy or plan, through
prototyping and other simulation methods. This process involves constant
improvement of the prototype as risks are decreased (while others may arise). Once
the prototype becomes suf ciently robust, and risk is reduced to acceptable levels,

the next step develops in accordance with the basic waterfall approach, through a
succession of stages: concept, requirements, design, and implementation (Boehm
1988 ). Once this cycle is concluded, another cycle begins, as a new increment of the
product is created.
The spiral model bears some resemblance to the incremental life cycle, but the
emphasis on risk evaluation presents a major difference. The stages or spirals that
constitute this model regard planning as a

first step, moving then to the exploration
of what the requirements are and subsequently calculating the risks. In this stage of
risk calculation, the model is structured to initiate a process of determination of
risks and of formulation of alternatives (Massey and Satao 2012 ); thus, risk man-
agement can be considered the centerpiece of the model (Fig. 2.3 ).
Boehm ( 2000 ) asserts that each cycle or iteration of the process will invariably
display six particular characteristics, which he named the “ invariants. ”