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CHAPTER 2 THE PROCESS 35

Evolutionary models are iterative. They are characterized in a manner that enables
software engineers to develop increasingly more complete versions of the software.

2.7.1 The Incremental Model
The incremental model combines elements of the linear sequential model (applied
repetitively) with the iterative philosophy of prototyping. Referring to Figure 2.7, the
incremental model applies linear sequences in a staggered fashion as calendar time
progresses. Each linear sequence produces a deliverable “increment” of the software
[MDE93]. For example, word-processing software developed using the incremental
paradigm might deliver basic file management, editing, and document production
The incremental model functions in the first increment; more sophisticated editing and document production
delivers software in capabilities in the second increment; spelling and grammar checking in the third
small but usable pieces, increment; and advanced page layout capability in the fourth increment. It should be
called “increments.” In
general, each increment noted that the process flow for any increment can incorporate the prototyping para-
builds on those that digm.
have already been When an incremental model is used, the first increment is often a core product.
delivered.
That is, basic requirements are addressed, but many supplementary features (some
known, others unknown) remain undelivered. The core product is used by the cus-
tomer (or undergoes detailed review). As a result of use and/or evaluation, a plan is
developed for the next increment. The plan addresses the modification of the core
product to better meet the needs of the customer and the delivery of additional
features and functionality. This process is repeated following the delivery of each
increment, until the complete product is produced.

System/information Increment 1
engineering
Analysis Design Code Test Delivery of
1st increment

Delivery of
Increment 2 Analysis Design Code Test
2nd increment

Increment 3 Analysis Design Code Test Delivery of
3rd increment

Increment 4 Analysis Design Code Test Delivery of
4th increment
FIGURE 2.7
The
incremental
model Calendar time