Iteration

80/20 Rule (Rule of Thumb)

• Also known as the Pareto principle.
• Roughly 80% of the effects come from 20% of the causes.
• Plan to specify 80% of requirements at start of project.
• 20% of code does 80% of work.

Iterative vs. Sequential

Iterative

• Iterative approaches are generally more useful; work better for most projects.
• Requirements are poorly understood or unstable.
• Design is complex or challenging.
• Team is unfamiliar with application.
  • Even worse when application is complex.
• Substantial risk.
• Long term predictability is not important.
• Cost of change is low.

Sequential

• Stable requirements (no need to iterate).
• Design is well understood.
• Team is familiar with application.
• Little risk.
• Long term predictability is important.
• When cost of later changes is high (ie. hardware).

Managing Change

• Studies at IBM show projects change ~25% during development.
• Check requirements for appropriateness.
• Explain cost of change.
  • Recall cost is not static throughout development process.
• Use a change control procedure.
• Cancel the project if doomed to fail.
• Determine if change is necessary for business.