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The waterfall model is a Software Development Life Cycle model (SDLC) and was the first process model to be introduced. The waterfall model is easy to understand and use, it is a stage gate model which means that the model consists of stages and gates. Each phase consists of one stage and one gate. Each phase has some pre-determined activities and requirements. The current phase must be completed before the next phase can begin and to end a phase it must pass a gate. A stage is the activities, and the gate is the requirements and the review that determine whether a project can continue.
The waterfall model consists of six different stages, 1) Requirements Gathering and Analysis, 2) Systems Development, 3) Systems Implementation and Coding, 4) Testing, 5) Deployment, 6) Systems Operations and Maintenance. 

The waterfall model is widely used for software development, but it is also a widely criticized model. The model is often criticized for the late detection of defects in the process and how to handle change requests among other things. It is most optimal to use the waterfall model for simple projects with clear reqirements and projects that do not change through the development process.

==About==
The first description of the Waterfall model was written by Winston W. Royce in 1970. Back then he did not call it the Waterfall model. <ref name="Large-Scale"> Petersen K., Wohlin C., Baca D. (2009) The Waterfall Model in Large-Scale Development. In: Bomarius F., Oivo M., Jaring P., Abrahamsson P. (eds) Product-Focused Software Process Improvement. PROFES 2009. Lecture Notes in Business Information Processing, vol 32. Springer, Berlin, Heidelberg. https://doi-org.proxy.findit.cvt.dk/10.1007/978-3-642-02152-7_29 </ref> Later it got the name because all the phases are done one by one and flows downwards like a waterfall. It is the finest and oldest known Software Development Life Cycle model (SDLC). There are defined and designed various Software Development Life Cycle models such as V-Shaped model, Big Bang Model, Iterative model etc. <ref name="Best Practices"> Kramer, Mitch, Best Practices in Systems Development Lifecycle: An Analyses Based on the Waterfall Model (2018). Review of Business & Finance Studies, v. 9 (1) p. 77-84, Available at SSRN: https://ssrn.com/abstract=3131958</ref> These models are also called Software Development Process Models and are all "''process consisting of a sequence of planned phases to develop or to amend the software products.''" <ref>Dubey A.; Jain A.; Mantri A., "Comparative Study: Waterfall v/s Agile Model", International Journal of Engineering Sciences & Research Technology (IJESRT), 2015 </ref>

=Application of the Waterfall model=
To be a good project manager it is important to be familiar with a list of different project life cycle models and be aware of where and when to use which project life cycle whether it is a predictive or an adaptive lifecycle.
To understand where and when the waterfall model potentially works best an adapted version of Stacey's matrix from Domain I: Agile Principles and Mindset by Sumanta Boral <ref name= "Agile Principles"> Boral S.,"Domain I: Agile Principles and Mindset", Ace the Pmi-acp pp. 1-27, 2016</ref> is used. 
The Stacey Matrix is published and developed by Ralph Douglas Stacey. It is developed to give an understanding of which factors there contributes to complexity and how to address the different degrees of complexity and with that knowledge choose the best suited project life cycle. <ref name="strategic management"> Stacey RD. Strategic management and organisational dynamics: the challenge of complexity. 3rd ed. Harlow: Prentice Hall, 2002 </ref>
The Stacey matrix is a two-dimensional matrix where uncertainty of the requirements are plottet against the perceived disagreement of the stakeholders.  <ref name= "Agile Principles"/>

[[File:WaterfallorAgile.png|thumb|right|500px|Figure 3: Application of the Waterfall model<ref name= "Agile Principles"/>]]


'''A project is close to certainty''' when "How" is well known for example if a similar project, decision or issue have been done before and therefor the effect and cause relationship is well known. Because then there will be some experiences and learnings there can be used to predict the "How" and set up the needed requirements from the beginning. <ref name= "Agile Principles"/> <ref name="strategic management"/>


'''A project is far from certainty''' if the project is new to the project manager for example if it is about something new or it is an innovative project. Then it can be hard to set up the needed requirements from the beginning and therefor there will be a lot of uncertainties about the "How" and the unclear relationship between cause and effect. <ref name= "Agile Principles"/> <ref name="strategic management"/>


'''A projects level of agreement''' is a measurement of the level of agreement about decisions and issues within the project group, the management team, and the stakeholders. This is important because the project manager function varies based on the level of agreement. <ref name= "Agile Principles"/> <ref name="strategic management"/>


The Waterfall model is placed in the bottom left corner in the zone that is called ''"simple"'' as illustrated on the figure. In this region the requirements are almost certain and there is a good consensus around the project. In this zone the decision making is based on reliable data gathered from the past to predict the future. <ref name= "Agile Principles"/>

=The waterfall model=
[[File:TheWaterfallModel.png|thumb|right|500px|Figure 1: Waterfall Model<ref name="Best Practices"/>]]
The waterfall model is a stage gate model which is a category of life cycle models. A stage gate model consists of multiple phases, each phase includes one stage and one gate, to get from one phase to another the project has to pass a gate. A stage consists of a description of activities there needs to be done during each state, the gate is the requirement and the review that determine whether a project can continue. The phases have to be processed in order without overlapping and within a specified timeframe. In the waterfall model it is not possible to move back and forth between phases, once a phase is completed it will be frozen and therefore it is not possible to move back and work on it again. The waterfall model consists of 6 phases, 1) Requirements Gathering and Analysis, 2) Systems Development, 3) Systems Implementation and Coding, 4) Testing, 5) Deployment, 6) Systems Operations and Maintenance. To achieve a positive outcome all six phases must be completed. <ref name="Best Practices"/>



==1. Requirements Gathering and Analysis==
===State===
In the first phase the customer's needs are identified and documented. Afterwards the identified needs have to be redefined as requirements to use them as input to the system development phase. The number of requirements can variate and depends on the available resources. Because when a new product is made it is not made from scratch only. It is typically made on top of parts of old products which are used as input to the requirement phase.  <ref name="Large-Scale"/>
In the first phase the project manager is planning the project by gathering the requirements from the organization and the stakeholders and begin the analysis to determine the project approach, deliverables, and anticipated outcomes to gather the non-functional and functional requirements. <ref name="Best Practices"/>

===Gate=== 
At this gate all the requirements must be checked whether they are understood, agreed upon and documented. It also have to be controlled whether all the relevant stakeholders are identified. <ref name="Large-Scale"/>

Deliverables: Functional and Business requirements and Requirements Understanding Document (RUD) <ref name="Best Practices"/>

==2. Systems Development==
===State===
This phase is in some descriptions of the waterfall model also called the design phase because this phase is all about describing the architecture of the system and document it. <ref name="Large-Scale"/> This can be done 
by converting all the detailed requirements, from the previous phase, into a completed detailed systems design. The detailed systems design is a detailed description of how the required functionalities (must- and wish- list)
can be delivered to the system which includes a precise description of the key components and the interface for the components.<ref name="Best Practices"/> 

===Gate===
This gate is a quality checklist that verifies the evaluation of the architecture, which includes checking for deviations between the design and requirements from the previous gate. Other than that there also have to be looked for deviations in the effort, planned timeline or product scope.<ref name="Large-Scale"/>  

Deliverables: Low-Level Design document (LLD) and High-Level Design Document (HLD)<ref name="Best Practices"/>

==3. Systems Implementation and Coding==
===State===
In this phase, all the information, requirements, and analysis gathered in the two previous phases are converted into code. This will show whether the previous phases is documented well enough for the developer to develop the application and implement the design. Depending on the size of the application this can be the longest phase of the waterfall model.  <ref name="Best Practices"/>
Before moving on to the next phase the developer conducts some basic unit tests to make sure that the application is running. <ref name="Large-Scale"/> 

===Gate===
This gate is about verifying that the developed application and design do not deviate from the requirements or agreements from previous states. <ref name="Large-Scale"/>  

Deliverables: Unit test cases, results and the application delivered by the developer. <ref name="Best Practices"/>

==4. Testing==
===State===
This phase involves testing and verifying that the application delivers the quality and functionality needed by performing an Integration test where the individual software modules are combined in a group and tested. <ref name="Large-Scale"/>
Other test methods can also be performed, such as use test cases, stress tests, module unit testing, Etc. It is crucial to track the progress when performing the tests. It is done by reporting all test activities, including the issues raised along the way.<ref name="Best Practices"/>

===Gate===
This gate is a review of the checklist to verify deviations between the application performance in terms of quality, requirements, and other terms from the previous gates. <ref name="Large-Scale"/>  

Deliverables: Test Cases, Test Reports, Defect Reports, Issues Log <ref name="Best Practices"/>

==5. Deployment==
===State===
Now that the application and design have passed the test phase it can now be deployed. 
In this phase, the application is presented to the end-user to ensure the application and design meet the needs and ensure that the specifications were right. 
It also proves whether the application and design perform as intended when placed in the intended environment. This kind of end-user validation test is called User Acceptance Testing (UAT).<ref name="Large-Scale"/> <ref name="Best Practices"/>
Another part of the deployment phase is to train the end-user to use the new application. 
It is important to log every issue during this state. It is acceptable if there occurs issues as long its not critical or high-level defects. If their occur any miner issues this will be handled in the Systems Operations and Maintenance phase.<ref name="Best Practices"/>

===Gate===
To get through this gate the application and design have to meet the customers' requirements, which includes its, presented in time, fulfils the customers' quality requirements, and the customer can accept the outcome.  <ref name="Large-Scale"/>

Deliverables: Environment Definition Specification and Issues Log <ref name="Best Practices"/>

==6. Systems Operations and Maintenance==
===State===
The last phase is when the application and design are ready to be released for the customer. After the release, some technical issues can occur, or the customer can discover new problems. Therefore it is essential to have an issue log open and conduct maintenance while releasing an application. Hiring technical and procedural experts is vital because the end-users will have questions. <ref name="Large-Scale"/> <ref name="Best Practices"/>

===Gate===
In this phase, there is no gate because the state is an ongoing process. There are some documents there can be a good idea to keep updated to keep an overview of the operation and maintenance. If a new application needs to be developed, The waterfall model can be used again from the beginning. 

Deliverables: User Manual, List of Production Tickets (Resolved), List of New Features Implemented

=Best Practice= 

''"The evolution of software that satisfies its user expectations is a necessary goal of a successful software development organization. To achieve this goal, software engineering practices must be applied throughout the evolution of the software product. Most of these software engineering practices attempt to create and modify software in a manner that maximizes the probability of satisfying its user expectations.”'' (James S. Collofello, 1998)<ref>James S. Collofello, (1998) “Introduction to Software Verification and Validation” SEI Curriculum Module SEI-CM-13-1.1, Carnegie Mellon University Software Engineering Institute</ref>


==Requirements Gathering and Analysis==

In this phase it would be a good idea to hire a Business Analyst to ensure that the functional requirements and business requirements meets the stakeholder's expectations. Business requirements is about determine the software user and how the software will be used. The business analyst will conduct a needs analysis together with the end user of the system. Because the end user knows or have an idea of which results, they expect and information they want from the system. In this state it is important to capture all the requirements, this can be done by using user or test cases, flowcharts, and current documentation. <ref name="Best Practices"/>  

Another best practice for the needs analysis is to arrange a meeting with the stakeholders either one on one or all together in a group to find out and understand their requirements, discussing any challenges and to receive all documentation on the current phase. It is important to make a documented outline of each process in collaboration with the stakeholders which include a complete description and understanding of their needs which can be separated into ''must have'' and ''wish list''. when making the documentation it is a good idea to use visuals like flowcharts, diagrams, and screenshots. It is important that the document include purpose, current processes, details of expected accomplishments and expectations. <ref name="Best Practices"/>   


== Systems Development ==
Business Architects would be helpful to hire in the Systems development phase. Business architects are specialists in producing designs of products. They know whether a developer can deliver the design or not. Therefore business architects can produce several functional designs from the requirements gathered in the first stage. These designs are then presented to the internal stakeholders, who can review the designs in a design document and approve the best design. After choosing the design, the business architects can finalize the design by creating the blueprint of the design, which includes all the necessary specifications about data resources, software, people, and information about coding and how to debug the application.<ref name="Best Practices"/>   

Best practice in the system development phase would be to describe the system characteristics and operations in detail by having a low-level and a high-level design document. These design documents include detailed descriptions of the desired characteristics of the software. It includes screen format maps, functional hierarchy diagrams, tables, business rules; business processes diagrams, pseudo-code, and an entity-relationship diagram of the data dictionary.<ref name="Best Practices"/>   
This design documentation is to assist the developer in the development process and make sure that additional input required will be held to a minimum. Another best practice is to include sign-off on all documents for the stakeholders.<ref name="Best Practices"/>

== Systems Implementation and Coding ==
Best practice in this phase is that the developer verifies that the application works according to the design specifications.<ref name="Best Practices"/>  

== Testing ==
In the testing phase, it is best practice to have the Business Architect or/and Analyst to review the issues log, Test Cases, test, and defect reports because they are the ones who provided the design specs and therefore also the ones who know what is intended and what to expect. If the application does not meet the exceptions from the Business Architect or/and Analyst, it needs to go back to the Developer to correct the issues. This verification process is a way to ensure that the application works as designed. <ref name="Best Practices"/>

== Deployment ==


== Systems Operations and Maintenance ==

=Pros and cons= 

=Ref=

Petersen K., Wohlin C., Baca D. (2009) The Waterfall Model in Large-Scale Development. In: Bomarius F., Oivo M., Jaring P., Abrahamsson P. (eds) Product-Focused Software Process Improvement. PROFES 2009. Lecture Notes in Business Information Processing, vol 32. Springer, Berlin, Heidelberg. https://doi-org.proxy.findit.cvt.dk/10.1007/978-3-642-02152-7_29

Kramer, Mitch, Best Practices in Systems Development Lifecycle: An Analyses Based on the Waterfall Model (2018). Review of Business & Finance Studies, v. 9 (1) p. 77-84, Available at SSRN: https://ssrn.com/abstract=3131958

http://tryqa.com/what-is-waterfall-model-advantages-disadvantages-and-when-to-use-it/

=References=
<references/>