* It is about WHAT not HOW

* It varies greatly. It may range from a high level abstract statement to a detailed mathematical specification

* It may be as complex as a 500 pages of description

* Ensures the user (not the developer) drives system functionalities

* Helps avoiding confusion and arguments

* Helps minimizing the changes which reduces the cost

Changes in requirements are expensive.

3 x as much during the design phase

5-10 x as much during implementation

10-100 x as much after release

Code Complete, p30

Average project experiences about 25% changes in the requirements

This accounts for 70-80% if the rework of the project Code Complete, p40

Important to plan for requirements changes

Example: the case of critical applications

* Usually the first attempt for the description of the requirements
* Includes services and constraints of the system
* Given in natural language or diagrams
* Readable by everybody, i.e. understandable by the user
* Serve business objectives

Example of User requirement:
As a user who found a new job announcement, I want to add a new position to the website so s/he can start working on doing the initial research and apply to it.

* Includes services and constraints of the system in detail
* Useful for the design and development
* Precise and cover all cases
* Structured presentation

Example of System requirement:
A registered user on the academic jobs website should be able to add a new position listing with the name of the school and academic unit, date of posting, date of expiry, application deadline, and contact and application details. The interaction fails if: the position is already listed, the application deadline is in the past, position announcement is expired, or the contact information is missing. If fails, point mistakes to user and ask the user to fix and resubmit.

* Lack of clarity - ambiguous language

* Confusion - design information are not distinguished between functional and non-functional requirements

* Several requirements are defined as a single one

* Incompleteness – requirements may be missing

* Inconsistency – requirements may contradict themselves

* Separate functional and non-functional requirements

* Requirements must be clearly identified by a number

* Include a rationale for each requirement – this helps clarify reasoning behind the requirements and may be useful for evaluating potential changes in the requirements

* Team should decide to use a standard form/template

* Have a clear way to establish and distinguish requirements priorities

* Write test cases; requirements must be testable

* Deliverables

* Elaborate the user requirements to get a precise, detailed and complete version of them

* Used by designers and developers

* Write system requirements using natural language

* Forms/templates and logic are used to bring some rigor, but make sure everyone understands them

Example: If sales for current month are below target sales, then report is to be printed unless difference between target sales and actual sales is less than half of difference between target sales and actual sales in previous month, or if difference between target sales and actual sales for the current month is less than 5%.

Problems: * Difficult to read * Ambiguity: 5% of what? * Incomplete: what if sales are above target sales?

Functional requirements
* Services the system should provide
* What the system should do or not do in reaction to particular situations

Non-functional requirements - might be more critical than functional ones
* Constraints on the services or functions offered by the system
* Examples: Timing constraints (e.g., one semester), constraints on the development process (CASE, language, development method…), standards etc

Domain requirements
* From the application domain of the system
* May be functional or non-functional
Examples: Medicine, library, physics, chemistry

Note: You can have all of user/system functional/nonfunctional requirements.

* Use case diagram shows all things an actor can do

* Can be broken down to individual user stories