Editing Waterfall model (section)

==Criticism==
Clients may not know the exact requirements before they see working software and thus change their requirements further on, leading to redesign, redevelopment, and retesting, and increased costs.<ref>{{cite journal |last1=Parnas |first1=David L. |last2=Clements |first2=Paul C. |title=A rational design process: How and why to fake it |journal=IEEE Transactions on Software Engineering|date=1986 |issue=2 |pages=251–257 |doi=10.1109/TSE.1986.6312940 |s2cid=5838439 |url=https://www.cs.tufts.edu/~nr/cs257/archive/david-parnas/fake-it.pdf |access-date=2011-03-21}}</ref>

Designers may not be aware of future difficulties when designing a new software product or feature, in which case revising the design initially can increase efficiency in comparison to a design not built to account for newly discovered constraints, requirements, or problems.<ref>{{cite book |last=McConnell |first=Steve |title=Code Complete, 2nd edition |publisher=Microsoft Press |year=2004 |isbn=1-55615-484-4 |url=https://archive.org/details/codecompleteprac00mcco }}</ref>

Organisations may attempt to deal with a lack of concrete requirements from clients by employing systems analysts to examine existing manual systems and analyse what they do and how they might be replaced. However, in practice, it is difficult to sustain a strict separation between [[systems analysis]] and programming,<ref>{{cite book|last=Ensmenger|first=Nathan|year=2010|title=The Computer Boys Take Over|url=https://archive.org/details/computerboystake00ensm|url-access=limited|isbn=978-0-262-05093-7|page=[https://archive.org/details/computerboystake00ensm/page/n50 42]|publisher=MIT Press }}</ref> as implementing any non-trivial system will often expose issues and edge cases that the systems analyst did not consider.

Some organisations, such as the United States Department of Defense, now have a stated preference against waterfall-type methodologies, starting with [[MIL-STD-498]] released in 1994, which encourages ''evolutionary acquisition'' and ''[[iterative and incremental development]]''.<ref>{{cite journal |last1=Larman |first1=Craig |last2=Basili |first2=Victir |title=Iterative and Incremental Development: A Brief History |journal=IEEE Computer |year=2003 |edition=June |url=http://doi.ieeecomputersociety.org/10.1109/MC.2003.1204375 |doi=10.1109/MC.2003.1204375 |volume=36 |issue=6 |pages=47–56|s2cid=9240477 |url-access=subscription }}</ref>