Incremental Model (IM)

This model, illustrated in Fig. 1.6, derives its name from the way in which the software is built. More specifically, the model is designed, implemented and tested as a series of incremental builds until the product is finished. A build consists of pieces of code from various modules that interact together to provide a specific function.

At each stage of the IM a new build is coded and then integrated into the structure, which is tested as a whole. Note that the product is only defined as finished when it satisfies all of its requirements.

This model combines the elements of the waterfall model with the iterative philosophy of prototyping. However, unlike prototyping the IM focuses on the delivery of an operational product at the end of each increment.

An example of this incremental approach is observed in the development of word processing applications where the following services are provided on subsequent builds:

1.      Basic file management, editing and document production functions

2.      Advanced editing and document production functions

3.      Spell and grammar checking

4.      Advance page layout

The first increment is usually the core product which addresses the basic requirements of the system. This maybe either be used by the client or subjected to detailed review to develop a plan for the next increment. This plan addresses the modification of the core product to better meet the needs of the customer, and the delivery of additionally functionality. More specifically, at each stage

·        The client assigns a value to each build not yet implemented

·        The developer estimates cost of developing each build

·        The resulting value-to-cost ratio is the criterion used for selecting which build is delivered next

Essentially the build with the highest value-to-cost ratio is the one that provides the client with the most functionality (value) for the least cost. Using this method the client has a usable product at all of the development stages.