Data Flow Diagrams (DFD)

Data Flow Diagrams (DFD): A Complete PMI-PBA Guide

What are Data Flow Diagrams (DFDs)?

Data Flow Diagrams (DFDs) are visual representations that show how data moves through a system or process. As a cornerstone of structured analysis techniques, DFDs map the flow of information, illustrating where data comes from, where it goes, how it's stored, and what processes transform it along the way.

Why DFDs are Important for Business Analysis

DFDs serve several critical purposes in business analysis:

• They provide a clear visualization of complex systems
• They help identify gaps, redundancies, and inefficiencies in data flows
• They create a common language for technical and non-technical stakeholders
• They support requirements elicitation and validation
• They serve as documentation for current systems and blueprints for future systems

Key Components of Data Flow Diagrams

DFDs consist of four primary elements:

1. External Entities (also called Terminators): Sources or recipients of data outside the system boundaries, represented by rectangles or squares. Examples include customers, suppliers, or other systems.

2. Processes: Activities that transform data, represented by circles or rounded rectangles. Each process should have inputs and outputs, showing how data is changed.

3. Data Stores: Repositories where data is held, represented by open-ended rectangles or parallel lines. These could be databases, files, or physical storage locations.

4. Data Flows: Pathways showing the movement of data, represented by arrows. Each arrow should be labeled to identify what data is moving.

Levels of DFDs

DFDs follow a hierarchical structure with different levels of detail:

• Context Diagram (Level 0): The highest-level view showing the system as a single process interacting with external entities

• Level 1 DFD: Expands the central process from the context diagram into multiple major processes

• Level 2+ DFDs: Further decompose processes from higher levels into more detailed subprocesses

DFD Rules and Best Practices

• Each process must have at least one input and one output
• Data cannot move directly from one external entity to another
• Data cannot move directly from one data store to another
• All flows must be labeled with meaningful data names
• Processes should be named with verbs and objects (e.g., "Validate Order")
• Lower-level DFDs must be balanced with their parent diagrams (maintain consistent inputs and outputs)
• Avoid overly complex diagrams with too many processes (7±2 is a good guideline)

Creating Effective DFDs

1. Identify scope and boundaries: Determine what's in and out of scope for your analysis

2. Identify external entities: List all sources and recipients of data

3. Start with a context diagram: Create the highest-level view

4. Define major processes: Break down the system into key functions

5. Identify data stores: Determine where data is kept

6. Map data flows: Show how data moves between components

7. Decompose into lower levels: Add detail as needed

8. Review and refine: Ensure logical consistency and completeness

DFD Notations

There are two main DFD notations:

1. Yourdon and Coad (Modern Structured Analysis) - Uses circles for processes

2. Gane and Sarson - Uses rounded rectangles with identifying information for processes

For PMI-PBA exam purposes, understanding both notations is beneficial, though you may see Gane and Sarson more frequently.

Exam Tips: Answering Questions on Data Flow Diagrams (DFD)

1. Know the symbols: Memorize the standard symbols for all DFD components and be able to distinguish between different notation styles.

2. Understand balancing: Questions may ask you to identify errors in balancing between parent and child diagrams. Remember that all inputs and outputs must be preserved across levels.

3. Recognize invalid flows: Be ready to spot improper connections like entity-to-entity or store-to-store flows that violate DFD rules.

4. Identify missing components: Practice spotting when a diagram is missing necessary elements like process inputs/outputs or flow labels.

5. Apply context awareness: Connect DFDs to other business analysis concepts like requirements, process modeling, and data modeling.

6. Look for appropriate detail: Assess whether the right level of decomposition is used for the given scenario.

7. Interpret diagrams holistically: Understand what the overall diagram is communicating about the system or process, not just the individual symbols.

8. Check for naming conventions: Verify that processes use verb-object naming, flows are properly labeled, and naming is consistent.

9. Consider stakeholder perspectives: Think about how different stakeholders would use and interpret the DFD information.

10. Connect to business value: Understand how DFDs support larger business analysis goals and project outcomes.

Sample Exam Question Approaches

Scenario-based questions: These often present a business situation and ask how you would model it using DFDs. Focus on identifying the key components (entities, processes, stores, flows) before selecting an answer.

Diagram interpretation: You may be shown a DFD and asked to identify problems, extract information, or determine what happens under certain conditions. Carefully trace data flows through the system.

Terminology questions: Know the precise definitions of DFD terms and components.

Application questions: Consider when and why you would use DFDs compared to other modeling techniques.

Remember that on the PMI-PBA exam, questions about DFDs will likely focus on their practical application in business analysis work rather than just theoretical knowledge. Focus on how DFDs support requirements analysis, stakeholder communication, and solution design.