Introduction to Requirements(Chapters 1-3 of the requirements text)

CSSE 371, Software Requirements and Specification

Don Bagert, Rose-Hulman Institute of Technology

September 2, 2004Thanks to Mark Ardis for some of the slides included.

Outline

• Requirements and the Cost of Errors

• Requirements Management

• Software Lifecycle Process Models

Requirements and the cost of errors

Biggest Reasons forProject Success and Failure

• Project Success– User involvement– Executive management support– Clear statement of requirements

• Project Failure– Lack of user input– Incomplete requirements– Changing requirements

Defects by Development Phase

• Requirements – 31% of Defects on Final Product Delivery

• Design – 25%• Coding – 12%• Documentation – 16%• Bad fixes – 16%

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Cost to Fix Errors

RequirementsDesignCodingUnit TestAcceptance TestMaintenance

1 5 25 50125500

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So…

• Requirements errors are likely to be the most common class of error

• Requirements errors are likely to be the most expensive to fix

Requirements Management

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What is a Software Requirement?

1. A software capability needed by the user to solve a problem to achieve an objective

2. A software capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification or other formally imposed documentation

[according to Dorfman and Thayer]

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What is Software Requirements Management?

• The process of systematically, eliciting, organizing and documenting requirements for the software system, and a process that establishes and maintains agreement between the customer and the project team on the changing requirements of the system.

Software Lifecycle Process Models

Waterfall Model

• Manufacturing life cycle model

• Assumes– multi-stage development cycle– completely separate stages– output of one stage is input for next– each stage complete before next is begun

Waterfall StagesRequirements

Design

Coding

Testing

Maintenance

Advantages

• Much better than chaos!

• Staged deliverables are possible

• Can modify model to allow for feedback

• Clearly defined stages help with – planning, scheduling– management, organization– accountability, control

Disadvantages

• Not very practical (do not know requirements in the beginning)

• No feedback, provision for modification

• Customer sees nothing until last step

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Spiral Model

• Incremental– more feedback

from customer

• Risk analysis– more rigorous

analysis of risk at each stage

Spiral Model

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Iterative (or Incremental) Approach

• Consists of four phases– Inception– Elaboration– Construction– Transition

• An iteration is a sequence of activities with an established plan and evaluation criteria, resulting in an executable of some type

• Each phase goes under a # of iterations

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Management

Environment

Business Modeling

Implementation

Test

Analysis & Design

Preliminary Iteration(s)

Iter.#1

Phases

Iterations

Iter.#2

Iter.#n

Iter.#n+1

Iter.#n+2

Iter.#m

Iter.#m+1

Deployment

Configuration Mgmt

Requirements

Elaboration TransitionInception Construction

Workflows group activities logically

In an iteration, you walk through all workflows

Process Workflow

Support Workflow

Rational Unified Process Model

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Team Project Process

• Waterfall model for this course• You may pick which model to use in CSSE

372 for implementation of the project