Consider Again the Example of the Three-step Airport Security:

This chapter is from the book 

1.4 Characteristics of a Good Requirement

A requirement needs to run across several criteria to be considered a "good requirement" [HUL05][LEF03] [LUD05][YOU01]. Good requirements should have the post-obit characteristics:

Unambiguous
Testable (verifiable)
Clear (concise, terse, unproblematic, precise)
Correct
Understandable
Feasible (realistic, possible)
Independent
Atomic
Necessary
Implementation-gratuitous (abstract)

Besides these criteria for private requirements, three criteria apply to the set of requirements. The set should exist

Consequent
Nonredundant
Consummate

The sample projection used in this book is an online travel agency, as shown in Effigy 1.2. You're probably familiar with this type of application considering variations of it tin can exist plant on several websites. The project is circuitous plenty to show possible relationships between various requirements types, but information technology is small enough to exist hands understood. Nigh of the examples in this chapter (and the other capacity) are related to this project.

Figure 1.2

Figure 1.2 The home page of an online travel agency.

Permit'due south discuss each of the criteria of a good requirement and prove some examples.

Unambiguous

There should exist simply 1 manner to translate the requirement. Sometimes ambiguity is introduced past undefined acronyms:

REQ1 The system shall be implemented using ASP.

Does ASP mean Active Server Pages or Application Service Provider? To fix this, nosotros can mention a full proper name and provide an acronym in parentheses:

REQ1 The system shall exist implemented using Active Server Pages (ASP).

Here's another example:

REQ1 The system shall not accept passwords longer than 15 characters.

It is non clear what the arrangement is supposed to do:

The arrangement shall not allow the user enter more 15 characters.
The system shall truncate the entered string to fifteen characters.
The system shall display an fault message if the user enters more than than 15 characters.

The corrected requirement reflects the clarification:

REQ1 The organization shall not accept passwords longer than 15 characters. If the user enters more than fifteen characters while choosing the password, an fault bulletin shall ask the user to right it.

Some ambiguity may be introduced through the placement of a certain word:

REQ1 On the "Stored Flight" screen, the user tin just view one record.

Does this mean that the user tin can "but view," not delete or update, or does information technology mean that the user can view only 1 tape, not two or three?

Ane style to gear up the problem is to rewrite the requirement from the arrangement'south point of view:

REQ1 On the "Stored Flight" screen, the system shall display only i flight.

Testable (Verifiable)

Testers should be able to verify whether the requirement is implemented correctly. The test should either pass or fail. To exist testable, requirements should be clear, precise, and unambiguous. Some words can make a requirement untestable [LUD05]:

Some adjectives: robust, condom, accurate, effective, efficient, expandable, flexible, maintainable, reliable, user-friendly, adequate
Some adverbs and adverbial phrases: quickly, safely, in a timely mode
Nonspecific words or acronyms: etc., and/or, TBD

Such a requirement might look something like this:

REQ1 The search facility should allow the user to find a reservation based on Last Name, Appointment, etc.

In this requirement, all search criteria should be explicitly listed. The designer and developer cannot estimate what the user ways by "etc."

Other problems tin be introduced by ambiguous words or phrasing:

Modifying phrases: as advisable, as required, if necessary, shall be considered
Vague words: manage, handle
Passive voice: the subject of the sentence receives the activeness of the verb rather than performing it
- REQ1 The airport lawmaking shall be entered past the user.
- REQ2 The drome code shall be entered.

The first example shows a archetype example of passive voice. In agile vocalisation it would read "The user shall enter the aerodrome code." Every bit the second example shows, another result of the apply of passive phonation is that the agent performing the activity is sometimes omitted. Who should enter this code—the system or the user?

Indefinite pronouns: few, many, near, much, several, any, anybody, anything, some, somebody, someone, etc.
- REQ1 The system shall resist concurrent usage by many users.

What number should be considered "many"—10, 100, 1,000?

Articulate (Concise, Terse, Simple, Precise)

Requirements should not contain unnecessary verbiage or information. They should be stated clearly and simply:

REQ1 Sometimes the user will enter Airdrome Code, which the system will understand, but sometimes the closest city may supersede information technology, so the user does not need to know what the airdrome code is, and information technology will still be understood by the organisation.

This sentence may be replaced past a simpler one:

REQ1 The organization shall identify the airdrome based on either an Airdrome Code or a City Name.

Correct

If a requirement contains facts, these facts should be true:

REQ1 Machine rental prices shall show all applicable taxes (including 6% state tax).

The tax depends on the country, so the provided 6% effigy is wrong.

Understandable

Requirements should exist grammatically right and written in a consistent style. Standard conventions should be used. The word "shall" should be used instead of "will," "must," or "may."

Feasible (Realistic, Possible)

The requirement should be doable within existing constraints such as time, money, and available resources:

REQ1 The system shall have a natural linguistic communication interface that will understand commands given in English language language.

This requirement may be not viable within a short span of development fourth dimension.

Independent

To understand the requirement, there should not be a demand to know any other requirement:

REQ1 The listing of available flights shall include flight numbers, departure fourth dimension, and inflow time for every leg of a flying.
REQ2 It should be sorted by price.

The give-and-take "Information technology" in the second sentence refers to the previous requirement. Yet, if the order of the requirements changes, this requirement volition not be understandable.

Diminutive

The requirement should contain a unmarried traceable element:

REQ1 The system shall provide the opportunity to book the flight, buy a ticket, reserve a hotel room, reserve a car, and provide information virtually attractions.

This requirement combines v atomic requirements, which makes traceability very hard. Sentences including the words "and" or "but" should exist reviewed to see if they can exist broken into atomic requirements.

Necessary

A requirement is unnecessary if

None of the stakeholders needs the requirement.

Removing the requirement will not impact the organization.

An example of a requirement that is not needed by a stakeholder is a requirement that is added past developers and designers because they assume that users or customers want it. For example, the fact that a developer thinks that users would like a feature that displays a map of the airport and he knows how to implement it is not a valid reason to add this requirement.

An example of a requirement that can exist removed because it does not provide whatsoever new information might wait like the following:

REQ1 All requirements specified in the Vision document shall be implemented and tested.

Implementation-gratis (Abstract)

Requirements should non contain unnecessary design and implementation information:

REQ1 Content information shall be stored in a text file.

How the information is stored is transparent to the user and should be the designer's or architect'due south determination.

Consistent

There should non be any conflicts between the requirements. Conflicts may exist direct or indirect. Direct conflicts occur when, in the same situation, different behavior is expected:

REQ1 Dates shall be displayed in the mm/dd/yyyy format.
REQ2 Dates shall be displayed in the dd/mm/yyyy format.

Sometimes it is possible to resolve the conflict by analyzing the conditions under which the requirement takes place. For case, if REQ1 was submitted by an American user and REQ2 by a French user, the preceding requirements may be rewritten equally follows:

REQ1 For users in the U.S., dates shall be displayed in the mm/dd/yyyy format.
REQ2 For users in France, dates shall exist displayed in the dd/mm/yyyy format.

This can eventually lead to the following requirement:

REQ3 Dates shall be displayed based on the format defined in the user'southward spider web browser.

Another example of a direct conflict tin exist seen in these ii requirements:

REQ1 Payment by PayPal shall exist available.
REQ2 Only credit bill of fare payments shall be accepted.

In this case the disharmonize cannot exist resolved past adding weather condition, so 1 of the requirements should exist changed or removed.

Indirect conflict occurs when requirements do not describe the aforementioned functionality, but it is not possible to fulfill both requirements at the same time:

REQ1 System should accept a natural language interface.
REQ2 System shall be developed in 3 months.

Some requirements do not disharmonize, but they use inconsistent terminology:

REQ1 For outbound and inbound flights, the user shall be able to compare flight prices from other, nearby airports.
REQ2 The outbound and render flights shall exist sorted by the smallest number of stops.

To draw the same concept, in the first requirement the term "inbound flights" is used, and in the 2d requirement the term "return flights" is used. The usage should be consequent.

Nonredundant

Each requirement should be expressed only once and should not overlap with another requirement:

REQ1 A calendar shall be available to assistance with inbound the flight date.
REQ2 The arrangement shall display a popular-up calendar when inbound any engagement.

The get-go requirement (related to only the flight date) is a subset of the second one (related to any engagement entered by the user).

Complete

A requirement should exist specified for all conditions that tin occur:

REQ1 A destination state does not need to exist displayed for flights within the U.S.
REQ2 For overseas flights, the arrangement shall brandish a destination state.

What about flights to Canada and Mexico? They are neither "within the U.S." nor"overseas."

All applicable requirements should be specified. This is the toughest condition to be checked. There is really no manner to be certain that all the requirements are captured and that one calendar week before the production date i of the stakeholders won't say, "I forgot to mention that I need one more characteristic in the application."

A good requirement should have more criteria. Still, they unremarkably tin can be expressed equally a combination of the criteria we accept merely discussed:

Modifiable: If it is atomic and nonredundant, it is usually modifiable.
Traceable: If it is atomic and has a unique ID, it is usually traceable.

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