See Methods and Standards - CLEANROOM method for the bigger picture.

This is a model for understanding what testing is required during a development -- and why. It also shows when the tests should be planned.

It's called "V" model because of the following diagram commonly used to present the proposal.

requirements       user                                        user
definition         requirements                          acceptance

systems                system                            system
analysis               proposal                            test

system	                  application          integration
design                     design docs                 test

construction                   program         unit test


What the diagram means is this:

user acceptance
Testing of the delivered system to prove it meets the user requirements. By its nature, the user acceptance test must be planned alongside the user requirements. User acceptance test scripts can be constructed by the users with assistance from the developers as the system is being constructed.
system test
Testing of the complete system, to prove it meets the system proposal. This enables the developers to prove to their satisfaction that the application is ready for delivery. The planning for this testing should take place alongside the production of the system proposal.
integration test
Tests all components within the system to ensure they work with each other correctly, thus proving the application design, as documented, has been met.
unit test
Testing individual code components, to prove each functions correctly according to the program specification. Unit testing should be planned alongside the construction of the program specification.

The model also requires checkpoints before proceeding from one stage to the next. For example, the user requirements are "tested" to ensure they provide the correct level of detail from which to produce the system proposal.

The first checkpoint is requirements testing. This occurs between requirements definition and systems analysis.

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