Chapter 5 Review Questions

Explain the relationship between a DFD level 0 diagram and a DFD level 1 diagram(s).

The DFD level 0 diagram depicts only the major high-level processes in the system. Each process is then decomposed into a more detailed DFD level 1 diagram. Typically a set of DFD diagrams for a system will include at least one DFD level 1 diagram for each process shown on the context level diagram.

Explain the relationship between a DFD context diagram and the DFD level 0 diagram.

A context level diagram is a high-level view of the entire system represented as one process symbol. Data flows going into and out of the system and the external entities that interact with the system are the only other element of the system shown on this type of diagram. A level 0 diagram is the first view of the internal processes of the business system. These processes are shown from fairly high level, with subsequent diagrams depicting increasing levels of detail. The level 0 diagram shows the same data flows and entities as the context, as well as data stores associated with the system.

What is meant by a DFD semantic error? Provide an example.

A semantics error is an error of meaning in the data flow diagram, in which the model does not accurately represent the business process being modeled. One example of a semantic error is one in which the data inputs to a process do not correspond to the type of information in the output data flow (see following DFD excerpt).

Why would one use a decision tree and/or a decision table in a process description?

Alternate methods of describing a process are a decision tree and a decision table. A decision tree is useful in that it aids in understanding decision logic pertaining to nodes (questions) and branches (answers). A decision table aids in understanding the actions (business policies) that based on a condition or a set of conditions.

Suppose your goal is to create a set of DFDs. How would you begin an interview with a knowledgeable user? How would you begin a JAD session?

Although my goal is to create a set of DFDs, it is best to begin by creating a set of use cases. Therefore, I would begin my interviews or the JAD session by working with the users to identify the major events that are handled by the system and to complete the use cases for those events. After I had a complete and verified set of use cases, then I could work on structuring the information from them into a set of DFDs for the system.

Suppose a process on a DFD is numbered 4.3.2. What level diagram contains this process? What is the process's parent process?

Level 2 diagram contains the process numbered 4.3.2 Parent process is 4.3

Define what is meant by a data flow in a process model. How should the data flow be named? What information about a data flow should be stored in the CASE repository?

A data flow represents a single piece of data or a set of logically-related data items that move to or from processes. A data flow should be named using a noun; information regarding a data flow to be stored in the CASE repository includes: • Label (name) • Type (flow) • Description • Alias • Composition (description of data elements) • Notes

Define what is meant by a data store in a process model. How should the data store be named? What information about a data store should be stored in the CASE repository?

A data store represents a set of data that is stored together - the data store holds the data. A data store should be named using a noun; information regarding a data store to be stored in the CASE repository includes: • Label (name) • Type (store) • Description • Alias • Composition (description of data elements) • Notes

Define what is meant by a process in a process model. How should the process be named? What information about a process should be stored in the CASE repository?

A process represents actions that are performed for some specific business reason. A process should be named using a verb phrase; information regarding a process to be stored in the CASE repository includes: • Label (name) • Type (process) • Description (what it is) • Process Number • Process Description (Structured English) • Notes

Define what is meant by an external entity in a process model. How should the external entity be named? What information about an external entity should be stored in the CASE repository?

An external entity is something that is outside the scope of our system, but interacts with it. An external entity may be a person, organization, or another system that supplies information to the system and/or receives information from the system. An external entity should be named using a noun; information regarding an external entity to be stored in the CASE repository includes: • Label (name) • Type (entity) • Description • Alias • Notes

Explain the process of balancing a set of DFDs.

Balancing involves assuring that all the information from a higher-level diagram is appropriately represented on the next level, or 'child' diagram. The child diagram shows more detail, but must show all the inflows and outflows to and from the parent process on the higher level diagram.

Distinguish between logical process models and physical process models.

Logical process models describe processes without suggesting how they are constructed. Physical process models provide information that is needed to ultimately build the system.

Discuss several ways to verify the correctness of a process model.

Model correctness can be verified by having the users validate it in a walkthrough, or through role-playing. Analysts should also check that the decomposition of diagrams goes to the same level of detail (not necessarily the same number of levels). Analysts should also check for consistency in terminology throughout the model.

Why is a process model typically composed of a set of DFDs? What is meant by decomposition of a business process?

Most business processes are too complex to depict using one diagram. Consequently, business processes are typically depicted with a set of DFDs, with the first diagram (Context Level) showing a summary of the system, and subsequent DFDs showing processes within that system. Decomposition is a method for breaking down a business process into smaller, logical processes.

How are mutually exclusive data flows (i.e., alternative routes thorough a process) depicted in DFDs?

Mutually exclusive data flows (alternatives) are all shown on the data flow diagram. Nothing is shown on the diagram to indicate these are alternatives. The information in the written process description will document the fact that the data flows are mutually exclusive.

What is a process model? What is a data flow model? Are the two related? If so, how?

Process models are used to document and organize the information obtained during the analysis phase of a systems development project. Data flow diagramming is a popular technique that is used in process modeling today. Data flow diagrams document the business processes and the data that passes among them.

Explain the use of structured English in process descriptions.

Structured English is one method of describing a process. Typically, short sentences are used to describe the elements or steps in the process

Identify three typical syntax errors commonly found in DFDs.

Students may list any of the syntax errors found in Figure 5-11.

Discuss how the analyst knows how to stop decomposing the process model into more and more levels of detail.

The complexity of an overall system or business process that is being modeled sets the boundary conditions for knowing what is the "ideal" level of decomposition for any specific system or process. Processes are typically decomposed into lower-level DFDs when the additional decomposition helps to explain the complexity of the process; experts suggest that each DFD page have at least three, but not more than seven to nine, processes. A good rule of thumb is to consider a process decomposition complete when the process in question can be explained on one sheet of paper (e.g., structured English, Pseudocode), or with one process description model (decision table/tree)

How can you make a DFD easier to understand? (Think first about how to make one difficult to understand).

Try to ensure that the processes are clearly named and that each process on a diagram is of about the same scale. It is confusing if some processes on a diagram are significantly more complex than others on the same diagram. Try to structure the flow through the processes from top to bottom, left to right, since that is the natural way most people read diagrams.

Creating use cases when working with users is a recent development in system analysis practice. Why is the trend today to employ use cases in user interviews or JAD sessions?

Users may be confused or intimidated by the symbols used in drawing DFDs. Most users can verbalize the content of a use case more easily than they can grasp drawing DFDs. Use cases are easier to start with than DFDs for most users.