What is an Iteration

An iteration is a set period of time within a project in which you produce a stable, executable version of the product, together with any other supporting documentation, install scripts, or similar, necessary to use this release. The executable is demonstrable, allowing the team to demonstrate true progress to stakeholders, get feedback on how they are doing so that they can improve their understanding of what needs to be done, and how to do it. Each iteration builds upon the results of previous iteration, and will produce a product increment one step closer to the final product. Iterations are timeboxed, meaning the schedule for an iteration should be regarded as fixed, and the scope of the iteration's content actively managed to meet that schedule.

At each iteration, artifacts are updated. It is said that this is a bit like "growing" software. Instead of developing artifacts one after another, in a pipeline fashion, they are evolving across the cycle, although at different rates.

Iterative development is very disciplined: the iteration length is fixed; the objectives of iterations are carefully planned; the evaluation criteria are established when each iteration is planned; and the tasks and responsibilities of participants are well defined. Additionally, objective measures of progress are captured. Some reworking takes place from one iteration to the next, but this too is done in a structured fashion.

Each iteration should address the most critical risks, and implement the highest-priority work items. This ensures that each iteration adds maximum stakeholder value, while reducing uncertainty. Iterative development is typically combined with frequent or continuous integration: as unit-tested components become available, they are integrated, then a build is produced and subjected to integration testing. In this way, the capability of the integrated software grows as the iteration proceeds, towards the goals set when the iteration was planned. Regular builds, such as daily or more frequent builds, let you break down the integration and test issues and spread them across the development cycle. These issues have often been the downfall of large projects because all problems were discovered at once during the single massive integration step, which occurred very late in the cycle, and where a single problem halts the whole team.

Why Iterate

The iterative approach has proven itself superior to the waterfall approach for a number of reasons:

• You are more likely to build an application that addresses user needs. Early specification of requirements often leads to unused features. The Standish Group has researched thousands of application development projects and has found that more than 45 percent of features are never used, while another 19 percent are used rarely  (see Figure 2.3). In other words, typically more than half of the development effort is wasted on developing nonessential capabilities. To avoid this problem, you need to involve the customer in the development project and use an iterative approach that allows you to implement and validate the capabilities deemed most essential in each iteration. This approach allows not only early validation of key capabilities but also addition of new capabilities late in the project.
• Integration is not one "big bang" at the end of a project. Leaving integration to the end results in time- and budget-consuming rework. To avoid this, an iterative approach breaks a project down into smaller iterations, each evolving executable code that is continuously integrated to enable rapid feedback and minimize later revision.
• Risks are usually discovered or addressed during early iterations. With the iterative approach, risks are more likely to be identified and addressed in early iterations. As an example, if there is a risk that a stakeholder will not be happy with the functionality you are developing, iterative development will encourage you to  implement the most essential capabilities partially and demonstrate them to key stakeholders to make sure that you are on the right track.
• Your ability to work effectively is fine-tuned. During early iterations, team members are walking through all lifecycle activities, from requirements capture and test definition to development, implementation, and testing. Consequently, they can make sure they have the tools, skills, organizational structure, and so on to work effectively.
• Management has a way of making tactical changes to the product. Management can make changes to the product along the way-to compete with other new products, for example. Iterative development allows you to evolve partial implementations of the end product quickly and use these for quick release of a reduced-scope product to counter a competitor's move.
• Reuse is facilitated. It is easier to identify common parts as they are being partially designed or implemented in iterations than to recognize them at the beginning. Discussions and reviews of the design in early iterations allow team members to spot potential opportunities for reuse and then develop a mature common code for these opportunities in subsequent iterations.
• Defects can be found and corrected over several iterations. This capability results in a robust architecture and a high-quality application. Flaws are detected in early iterations, rather than during a massive testing phase at the end. Performance bottlenecks are discovered while they can still be addressed instead of creating panic on the eve of delivery.
• Project personnel are better used. Many organizations match their use of a waterfall approach with a pipeline organization: the analysts send the completed requirements to designers, who send a completed design to programmers, who send components to integrators, who send a system to testers. These many handoffs are sources of errors and misunderstandings and make people feel less responsible for the final product. An iterative process encourages widening the scope of expertise of the team members, allowing them to play many roles and thus enabling a project manager to make better use of the available staff and simultaneously remove problematic handoffs.
• Team members learn along the way. The project members have several opportunities within a development cycle to learn from their mistakes and improve their skills from one iteration to another. More training opportunities can be discovered as a result of assessing the earlier iterations.
• The development process itself is improved and refined along the way. The iteration assessment allows the team to not only look at the project status from a product or scheduling perspective but also analyze what can be improved in the next iteration in both the organization and the process. One technique for doing so is to hold a retrospective.

Figure 2.3. Most Features Implemented Are Never or Rarely Used.
According to the Standish Group, an amazing 45 percent of features implemented are never used, while another 19 percent are used only rarely. If features never used were not implemented in the first place, development time would be cut in about half. Further, since productivity is typically measured in the form of lines of code or functionality delivered, this improvement would not register as a productivity increase using standard productivity measures.

Iteration Length

Iterations are typically 4 weeks long, although some teams will work with iterations as short as a week or as long as six weeks. For factors driving iteration length, see Table 1.

Table 1. Factors Impacting Iteration Length.