Butterflies are composed of three pieces - two wings and a body. Each part represents a piece of software testing, as under.
► Test Analysis
► Test Design
► Test Execution
From the testing point of view, all of the models are deficient in various ways:
The Test activities first start after the implementation. The connection between the various test stages and the basis for the test is not clear.
The tight link between test, debug and change tasks during the test phase is not clear.
Many of the process models currently used can be more generally connected by the 'V' model where the 'V' describes the graphical arrangement of the individual phases. The 'V' is also a synonym for Verification and Validation.
By the ordering of activities in time sequence and with abstraction levels the connection between development and test activities becomes clear. Oppositely laying activities complement one another (i.e.) server as a base for test activities. For example, the system test is carried out on the basis of the results specification phase.
The waterfall model is a sequential software development process, in which progress is seen as flowing steadily downwards (like a waterfall) through the phases of Conception, Initiation, Analysis, Design (validation), Construction, Testing and Maintenance.
Top-down and bottom-up are strategies of information processing and knowledge ordering, mostly involving software, but also other humanistic and scientific theories (see systemics). In practice, they can be seen as a style of thinking and teaching. In many cases top-down is used as a synonym of analysis or decomposition, and bottom-up of synthesis.
In the Spiral Model, a cyclical and prototyping view of software development is shown. Test are explicitly mentioned (risk analysis, validation of requirements and of the development) and the test phase is divided into stages. The test activities include module, integration and acceptance tests. However, in this model the testing also follows the coding. The exception to this is that the test plan should be constructed after the design of the system. The spiral model also identifies no activities associated with the removal of defects.