The waterfall model is a software development model in which development is seen as flowing steadily through the phases of requirements analysis, design, implementation, software testing (validation), integration, and software maintenance. The term was introduced in 1970 by W. W. Royce, who advocated an iterative approach to software development.

=Typical waterfall stages=

*Concept *Requirements definition *System and software design *Implementation and unit testing *Integration and system testing *Maintenance

=Pejorative Use=

Despite Royce s intentions, waterfall model has since come to refer to an inflexible, non-iterative approach to software development. When used pejoratively, the term criticizes the naivety of following a unidirectional flow, seen as unsuitable for a real world process. In most software development methodologies, Iterations (returning to or adapting results of the preceding stage) are common.

In summary, the criticisms of a non-iterative approach are as follows: *Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. *No formal means of exercising management control over a project and planning control and risk management are not covered within the model itself. *The lifecycle can take so long that the original requirements may no longer be valid by the time the system is implemented. *Estimating time and costs is difficult for each stage.

=See also=

*Project lifecycle *Spiral model *Chaos model *Agile software development

=References=

=External links=