Requirements Analysis - Types of Requirements

Types of Requirements

Requirements are categorized in several ways. The following are common categorizations of requirements that relate to technical management:

Customer Requirements

Statements of fact and assumptions that define the expectations of the system in terms of mission objectives, environment, constraints, and measures of effectiveness and suitability (MOE/MOS). The customers are those that perform the eight primary functions of systems engineering, with special emphasis on the operator as the key customer. Operational requirements will define the basic need and, at a minimum, answer the questions posed in the following listing:

• Operational distribution or deployment: Where will the system be used?
• Mission profile or scenario: How will the system accomplish its mission objective?
• Performance and related parameters: What are the critical system parameters to accomplish the mission?
• Utilization environments: How are the various system components to be used?
• Effectiveness requirements: How effective or efficient must the system be in performing its mission?
• Operational life cycle: How long will the system be in use by the user?
• Environment: What environments will the system be expected to operate in an effective manner?

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.

Behavioral Requirements

Behavioral requirements explain what has to be done by identifying the necessary behavior 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 toplevel 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 behaviors.

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 characterized in terms of the degree of certainty in their estimate, the degree of criticality to system success, and their relationship to other requirements.

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.

Well-known requirements categorization models include FURPS and FURPS+, developed at Hewlett-Packard.