1. Lecture 5: Requirements Engineering
Dr Valentina Plekhanova
University of Sunderland, UK

2. What is a Requirement? [Pfleeger, 1998]
A requirement is a feature of the system or a description of something the system is capable of doing in order to fulfil the system’s purpose.
Lecture 5
Valentina Plekhanova

3. What is a Requirement?
It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification.
May be the basis for a bid for a contract - therefore must be open to interpretation.
May be the basis for the contract itself - therefore must be defined in detail.
Both these statements may be called requirements.
Lecture 5
Valentina Plekhanova

4. Wicked Problems [Sommerville; Vliet]
Originally this term was defined by Rittel and Webber, in 1973.
A “wicked problem" is a complex problem, i.e. this problem is very difficult to define and/or identify (e.g. define related entities, definitive problems specification).
In general, no one can accurately predict where/when we can expect the problem and what effect it will have on the software production, etc.
The problem can be tackled after it has happened.
Lecture 5
Valentina Plekhanova

5. Wicked Problems Most large software systems address wicked problems.
Problems which are so complex that they can never be fully understood and where understanding develops during the system development.
Therefore, requirements are normally both incomplete and inconsistent.
Lecture 5
Valentina Plekhanova

6. A Stakeholder ..!!!
The notion of stakeholder is used to refer to anyone who should have some influence on the system requirements, e.g. end-users, engineers, business managers, domain experts, etc.
Lecture 5
Valentina Plekhanova

7. Some Reasons for Inconsistency
Different stakeholders have different requirements and they may define these in different ways.
There is a constantly shifting compromise in the requirements...
Different people may negotiate different parts.
Lecture 5
Valentina Plekhanova

8. Some Reasons for Incompleteness
Requirements can be interpreted differently.
Many requirements can be identified clearly only after some experience with the system.
Too many details would need to be specified.
Customer asks for too little.
The world changes.
It is difficult to define the level of precision in the requirements statements.
Informal language (e.g. natural language) does not help in requirements statements.
Lecture 5
Valentina Plekhanova

9. Requirements: Functional and Non-Functional
Functional requirements describe system services or functions.
Non-functional requirements is a constraint on the system or on the development process, e.g. timing constraints, standards, etc.
Lecture 5
Valentina Plekhanova

10. Non-Functional Requirements
Process Requirements:
standards, delivery,etc.
External Requirements: ethical, legislative
Efficiency Requirements: efficiency, reliability, portability,usability
Lecture 5
Valentina Plekhanova

11. Requirements Engineering
The process of establishing the services that
the customer requires from a system …
under the constraints with which it operates ...
and is developed!
Lecture 5
Valentina Plekhanova

12. The Requirements Engineering Process
Feasibility Study
Requirements Validation
Requirements Definition 1 5 3 2 4
Requirements Analysis
Requirements Specification
Lecture 5
Valentina Plekhanova

13. 1. Feasibility study 1
Find out if the current user needs can be satisfied given the available technology and budget?
A definition of the problems.
Alternative solutions and their expected benefits.
Required resources, costs, time and delivery dates for each alternative solution.
Lecture 5
Valentina Plekhanova

14. 2. Requirements Analysis
Find out what system stakeholders require from the system.
Lecture 5
Valentina Plekhanova

15. 3. Requirements Definition 4. Requirements Specification….
The ultimate purpose of the requirements-related activities is:
To understand the goals of the system;
To document the requirements to be met;
To specify the qualities required of the software solution: functionality, performance, reliability, etc.
Lecture 5
Valentina Plekhanova

16. 3. Requirements Definition
A statement in natural language plus diagrams of the services the system provides and its operational constraints.
Define the requirements in a form understandable to the customer.
It represent an understanding between the customer and developer of what the customer needs/wants.
It is usually written jointly by the customer and developer.
Written for customers.
Lecture 5
Valentina Plekhanova

17. 4. Requirements Specification
A structured document setting out detailed descriptions of the system services.
Define the requirements in detail.
It uses technical terms to define the system services: a very precise, even mathematical description – but it must be understandable by the client.
Written as a contract between client and contractor.
Lecture 5
Valentina Plekhanova

18. Software Specification
A detailed software description which can serve as a basis for a design or implementation.
Written for developers.
Lecture 5
Valentina Plekhanova

19. An Example of: Requirements Document Structure (1)
Introduction
Describe need for the system and how it fits with business objectives.
Glossary
Define technical terms used.
System models
Define models showing (high level architectural) system components and relationships.
Lecture 5
Valentina Plekhanova

20. Requirements Document Structure (cont 2)
Non-functional requirements definition
Define constraints on the system and the development process.
System evolution
Define fundamental assumptions on which the system is based and anticipated changes.
Requirements specification
Detailed specification of functional requirements.
Lecture 5
Valentina Plekhanova

21. Requirements Document Structure (cont 3)
Appendices
System hardware platform description.
Database requirements (as an ER model perhaps).
Index
More examples of requirements document can be found in [Pfleeger, 1998; Sommerville]
Lecture 5
Valentina Plekhanova

22. 5. Requirements Validation
Concerned with demonstrating the requirements define the system that the customer really wants.
Requirements error costs are high so validation is very important ...
Fixing a requirements error after delivery may cost up to 100 times the cost of fixing an implementation error.
Prototyping is an important technique of requirements validation.
Lecture 5
Valentina Plekhanova

23. Requirements Checking
Validity – Does the system provide the functions which best support the customer’s needs?
Consistency – Are there any requirements conflicts?
Completeness – Are all functions required by the customer included?
Realism – Can the requirements be implemented given available budget and technology?
Lecture 5
Valentina Plekhanova

24. Key Points
It is very difficult to formulate a complete and consistent requirements specification.
A requirements definition, a requirements specification and a software specification are ways of specifying software for different types of reader.
The requirements document is a description for customers and developers.
Requirements errors are usually very expensive to correct after system delivery.
Reviews involving client and contractor staff are used to validate the system requirements.
Stable requirements are related to core activities of the customer for the software. 
Volatile requirements are dependent on the context of use for the system.
Lecture 5
Valentina Plekhanova

25. Week 8: 24.04.2003-28.04.2003 Project Control Session
Tutorial Time: 10 minutes for each Team
Students will present project file, particularly Schedule, plus any project documentation.
Students will describe where they are in the project and any problems encountered.
During the discussion reviewers will ask to see evidence of deliverables for any tasks that are complete to determine whether they have in fact been done.
Lecture 5
Valentina Plekhanova