Software Engineering-Quality Function Deployment

Quality function deployment (QFD) is a quality management technique that translates the needs of the customer into technical requirements for software. Originally developed in Japan and first used at the Kobe Shipyard of Mitsubishi Heavy Industries, Ltd., in the early 1970s, QFD “concentrates on maximizing customer satisfaction from the software engineering process [ZUL92].” To accomplish this, QFD emphasizes an understanding of what is valuable to the customer and then deploys these values throughout the engineering process. QFD identifies three types of requirements:

Normal requirements. The objectives and goals that are stated for a product or system during meetings with the customer. If these requirements are present, the customer is satisfied. Examples of normal requirements might be requested types of graphical displays, specific system functions, and defined levels of performance.

Expected requirements. These requirements are implicit to the product or system and may be so fundamental that the customer does not explicitly state them. Their absence will be a cause for significant dissatisfaction. Examples of expected requirements are: ease of human/machine interaction,
overall operational correctness and reliability, and ease of software installation.

Exciting requirements. These features go beyond the customer’s expectations and prove to be very satisfying when present. For example, word processing software is requested with standard features. The delivered product contains a number of page layout capabilities that are quite pleasing and

In actuality, QFD spans the entire engineering process [AKA90]. However, many QFD concepts are applicable to the requirements elicitation activity. We present an overview of only these concepts (adapted for computer software) in the paragraphs that follow.

 In meetings with the customer, function deployment is used to determine the value of each function that is required for the system. Information deployment identifies both the data objects and events that the system must consume and produce. These are tied to the functions. Finally, task deployment examines the behavior of the system or product within the context of its environment. Value analysis is conducted to determine the relative priority of requirements determined during each of the three deployments.

QFD uses customer interviews and observation, surveys, and examination of historical data (e.g., problem reports) as raw data for the requirements gathering activity. These data are then translated into a table of requirements—called the customer voice table—that is reviewed with the customer. A variety of diagrams, matrices, and evaluation methods are then used to extract expected requirements and to attempt to derive exciting requirements.


As requirements are gathered as part of informal meetings, FAST, or QFD, the software engineer (analyst) can create a set of scenarios that identify a thread of usage for the system to be constructed. The scenarios, often called use-cases, provide a description of how the system will be used.

To create a use-case, the analyst must first identify the different types of people (or devices) that use the system or product. These actors actually represent roles that people (or devices) play as the system operates. Defined somewhat more formally, an actor is anything that communicates with the system or product and that is external to the system itself.

It is important to note that an actor and a user are not the same thing. A typical user may play a number of different roles when using a system, whereas an actor represents a class of external entities (often, but not always, people) that play just one role. As an example, consider a machine operator (a user) who interacts with the control computer for a manufacturing cell that contains a number of robots and numerically controlled machines. After careful review of requirements, the software for the control computer requires four different modes (roles) for interaction: programming mode, test mode, monitoring mode, and troubleshooting mode. Therefore, four actors can be defined: programmer, tester, monitor, and troubleshooter. In some cases, the machine operator can play all of these roles. In others, different people may play the role of each actor.

Because requirements elicitation is an evolutionary activity, not all actors are identified during the first iteration. It is possible to identify primary actors [JAC92] during the first iteration and secondary actors as more is learned about the system. Primary actors interact to achieve required system function and derive the intended benefit from the system. They work directly and frequently with the software. Secondary actors support the system so that primary actors can do their work.

Once actors have been identified, use-cases can be developed. The use-case describes the manner in which an actor interacts with the system. Jacobson  suggests a number of questions that should be answered by the use-case:
What main tasks or functions are performed by the actor?
What system information will the actor acquire, produce, or change?
Will the actor have to inform the system about changes in the external environment?
• What information does the actor desire from the system?
• Does the actor wish to be informed about unexpected changes?

In general, a use-case is simply a written narrative that describes the role of an actor as interaction with the system occurs.

Recalling basic SafeHome requirements , we can define three actors: the homeowner (the user), sensors (devices attached to the system), and the monitoring and response subsystem (the central station that monitors SafeHome). For the purposes of this example, we consider only the homeowner actor. The homeowner interacts with the product in a number of different ways:

enters a password to allow all other interactions
inquires about the status of a security zone
inquires about the status of a sensor
presses the panic button in an emergency
activates/deactivates the security system

A use-case for system activation follows:

1. The homeowner observes a prototype of the SafeHome control panel  to determine if the system is ready for input. If the system is not ready, the homeowner must physically close windows/doors so that the ready indicator is present. [A not ready indicator implies that a sensor is open; i.e., that
a door or window is open.]

2. The homeowner uses the keypad to key in a four-digit password. The password is compared with the valid password stored in the system. If the password is incorrect, the control panel will beep once and reset itself for additional input. If the password is correct, the control panel awaits further

3. The homeowner selects and keys in stay or away  to activate the system. Stay activates only perimeter sensors (inside motion detecting sensors are deactivated). Away activates all sensors.

When activation occurs, a red alarm light can be observed by the homeowner. 

Use-cases for other homeowner interactions would be developed in a similar manner. It is important to note that each use-case must be reviewed with care. If some element of the interaction is ambiguous, it is likely that a review of the use-case will indicate a problem.

Each use-case provides an unambiguous scenario of interaction between an actor and the software. It can also be used to specify timing requirements or other constraints for the scenario. For example, in the use-case just noted, requirements indicate that activation occurs 30 seconds after the stay or away key is hit. This information can be appended to the use-case.

Use-cases describe scenarios that will be perceived differently by different actors. Wyder  suggests that quality function deployment can be used to develop a weighted priority value for each use-case. To accomplish this, use-cases are evaluated from the point of view of all actors defined for the system. A priority value is assigned to each use-case (e.g., a value from 1 to 10) by each of the actors. An average priority is then computed, indicating the perceived importance of each of the usecases. When an iterative process model is used for software engineering, the priorities can influence which system functionality is delivered first.
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