Incremental Model

The incremental model combines elements of the linear sequential model (applied repetitively) with the iterative philosophy of prototyping. Referring to the fig. below, the incremental model applies linear sequences in a staggered fashion as calendar time progresses. Each linear sequence produces a deliverable “increment” of the software. For example, word-processing software developed using the incremental paradigm might deliver basic file management, editing and document production capabilities in the second increment; spelling and grammar checking in the third increment; and advanced page layout capability in the fourth increment. It should be noted that the process flow for any increment can incorporate the prototyping paradigm.

When an incremental model is used, the first increment is often a core product. That is, basic requirements are addressed, but many supplementary features (some known, others unknown) remain undelivered. The core product is used by the customer (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.

The incremental process model, like prototyping and other evolutionary approaches, is iterative in nature. But unlike prototyping, the incremental model focuses on the delivery of an operational product with each increment. Early increments are stripped down versions of the final product, but they do provide capability that serves the user an also provide a platform for evaluation by the user.

Incremental development is particularly useful when staffing is unavailable for a complete implementation by the business deadline that has been established for the project. Early increments can be implemented with fewer people. If the core product is well received, then additional staff (if required) can be added to implement the next increment. In addition, increments can be planned to manage technical risks. For example, a major system might require the availability of new hardware that is under development and whose delivery date id uncertain. It might be possible to plan early increments in a way that avoids the use of this hardware, thereby enabling partial functionality to be delivered to end-users without inordinate delay.

Advantages
• Early increments can be developed with few people.
• It combines iterative nature of prototyping model and linear nature of Linear Sequential Model.
• Number of people required is less
• Easy to add quality
• The system can be designed in such a manner that it can be delivered into pieces.
• Increments are developed one after the other, after feedback has been received from the user.
• Since each increment is simpler than the original system, it is easier to predict resources needed to accomplish the development task within acceptable accuracy bounds.
• Increments can be planned to manage technical risks.
• Deadlines can be managed in an effective manner.

Disadvantages
• Reusability of codes among the modules is minimum.
• Integration Testing is difficult to do.
• Each quality developed requires testing.
• It can be applied to only those projects which have independent modules.