Requirements Analysis

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The process of capturing the requirements for the system-to-be.


The dish




Initially, it may be a good idea just to capture the requirements as individual requirements using the EARS Requirement Capture technique.

As the General Analysis progresses, try to organise the captured requirements into groups as mentioned below in the Why section.

After organising the requirements, assign unique ID's to each requirement.

The terms used are important:

  • "Shall" - the verb 'shall' states that a requirement must be implemented
  • "Will" - a statement containing 'will' identifies a future happening
  • "Must" - requirements identify strong wishes from the customer, but not requirements
  • "To be", "is to be", "are to be", "should" and "should be" are usually not used in requirements as they identify considered capabilities of the system-to-be.

By having the customer answer the questions posed in the following listing, a deeper knowledge can be achieved:

  • Where will the system be used?
  • How will the system accomplish its mission objective?
  • What are the critical system parameters to accomplish the mission?
  • How are the various system components to be used?
  • How effective or efficient must the system be in performing its mission?
  • How long will the system be in use by the user?
  • What environments will the system be expected to operate in an effective manner?

To locate the needs, you have to listen carefully to the customer's storytelling. A good tool is to draw a Rich Picture to get an understanding of the functionality. Then you analyse the information to get a structured picture of the problem. This analysis may include Preliminary Use Cases, System Definition and even Prototyping. Then you make a description and present it to the customer in order to secure that you have understood his needs correctly. You may have to go through this process several times before you are able to locate all the exact requirements.

It is important that the requirements are:

1. unambiguous

2. complete

3. consistent

4. correct

5. measurable & testable

6. changeable

7. traceable

8. documented

9. actionable

10. defined to a level of detail sufficient for system design.

in order to make the verification and product accept easy and without conflicts.

See [1] for a more detailed explanation.


Requirements are the specification that determines what the system-to-be must conform to in order to fulfil the customer's needs or wishes to the system-to-be, i.e. the customer's needs are transformed into requirements for the system-to-be enabling the system-to-be to fulfil the needs.

While working with the other activities in the General Analysis part, requirements emerge and should be captured promptly.

The initial requirements can be located in the artefacts produced in the PreProject phase. A story contains a lot of implicit requirements; the Rich Picture may embed requirements, etc.

Very often, customers cannot present a detailed set of specifications of the system-to-be, but rather inform the developer how the system-to-be is expected to work and react in an unstructured manner. The developer must transform these expectations into a more formal description setting up exact requirements for the system-to-be.

We have to realise that it is a complicated and iterative process to identify the customer's needs. Especially, the software can be highly abstract reasoning which might be difficult to understand for the customer. A process has input data, activity and output data. The pattern to locate the exact requirements is therefore:

  • Input data: The customer's description of his vision, his needs and the concept for the project.
  • Activity: Run through these activities repeatedly: Capture Requirements -> Analyse Requirements -> Specify Requirements -> Verify Requirements
  • Output data: The result from the process will be a description of the exact requirements.

Requirements can be depicted in a SysML diagram suited for this purpose. Related requirements can be collected in packages. Each package contains a separate SysML diagram with the specific requirements.

Requirements can be grouped into related requirements specifying different sides of the product.

Requirement types

Below is a quite comprehensive list of requirements types or grouping. Not all requirement types are relevant in all project types.

Customer Requirements/Customer Needs/User Needs

Statements of fact and assumptions that define the expectations of the system in terms of mission objectives, environment, constraints and Measures Of Effectiveness and Measures Of Suitability (MOE/MOS). This type of requirements is typically expressed in a informal way and represents the customer's expectations to the system-to-be. It is these requirements that must be transformed into requirements for the system-to-be enabling it to fulfil the customer's needs.

Behavioural Requirements

Behavioural requirements explain what has to be done by identifying the necessary behaviour of a system.

Functional Requirements

Functional requirements explain what has to be done by identifying the necessary task, action or activity that must be accomplished. Functional requirements analysis will be used as the top-level functions for functional analysis.

Non-functional Requirements

Non-functional requirements are requirements that specify criteria that can be used to judge the operation of a system, rather than specific behaviours.

Performance Requirements

The extent to which a mission or function must be executed; generally measured in terms of quantity, quality, coverage, timeliness or readiness. During requirements analysis, performance (how well does it have to be done) requirements will be interactively developed across all identified functions based on system life cycle factors, and characterised in terms of the degree of certainty in their estimate, the degree of criticality to system success and their relationship to other requirements.

Architectural Requirements

Architectural requirements explain what has to be done by identifying the necessary system architecture of a system.

Structural Requirements

Structural requirements explain what has to be done by identifying the necessary structure of a system.

Design Requirements

The "build to", "code to" and "buy to" requirements for products and "how to execute" requirements for processes expressed in technical data packages and technical manuals.

Derived Requirements

Requirements that are implied or transformed from higher-level requirement. For example, a requirement for long-range or high-speed may result in a design requirement for low weight.

Allocated Requirements

A requirement that is established by dividing or otherwise allocating a high-level requirement into multiple lower-level requirements. Example: A 100-pound item that consists of two subsystems might result in weight requirements of 70 pounds and 30 pounds for the two lower-level items.

Source Types of Requirements as of 2011 02 27

Typical used Reuqirement Types

It is typical to at least include

  • Functional Requirements ("system shall do <requirement>")
  • Non-functional Requirements ("system shall be <requirement>")
  • Behavioural Requirements ("how the system shall react")
  • Performance Requirements ("how well does it have to be done")

from the list above, but select carefully according to the particular project.


Refer to Product Acceptance and Verification in order to get information about how to verify the requirements. It can be a good idea to specify the verification at the same time as the requirements are written. If it is difficult to write the verification, it may be due to an inadequately specified requirement.

A thorough explanation of the requirements capture process can be found through this link



Packages with requirements


One package with an incomplete requirements diagram