Iterative and incremental model

The iterative and incremental model builds the system in repeated cycles (iterations), delivering the product in small parts (increments). Rather than completing everything in one pass as in waterfall, a basic version is built first and then improved and extended in later cycles.

• Iterative: repeat the development cycle to refine and improve the same parts (rework based on feedback).
• Incremental: add new functionality in each cycle, so the system grows.

 Increment 1:  Req -> Design -> Code -> Test -> Release 1
 Increment 2:  Req -> Design -> Code -> Test -> Release 2
 Increment 3:  Req -> Design -> Code -> Test -> Release 3
                                         (full system)

Working

1. Collect and outline overall requirements; set priorities.
2. Choose the most important features for the first increment.
3. Design, develop and test that increment.
4. Deploy it to users and obtain feedback.
5. Plan the next increment including changes and new features.
6. Repeat until the complete system is delivered.

Advantages

• Working software is available early, so users give feedback early.
• Easier to accommodate changes in requirements.
• Risk is reduced as problems appear in earlier iterations.
• Testing and debugging are done on small pieces.
• Lower initial cost; customer benefit earlier.

Disadvantages

• Needs good planning and design to integrate increments.
• Overall architecture issues may appear later.
• Total cost may rise because of rework.
• More management effort and documentation tracking.

Example

A college ERP is released in increments: first admission, then examination, then library and accounts. The college starts using the admission module within two months.

Agile, spiral and RUP are examples of iterative-incremental approaches.