ACG 4401

Data Modeling:Data Flow DiagramsFlow Charts

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Benefits of DFD’s

• Uncover misunderstandings of system processes• Help communicate analyst system understanding to

management/end users• Helps to take the system out of context

– Eases the problem of thinking outside of the box

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Fed Ex Process (Physical DFD)

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DFD Guidelines

1. Choose meaningful names for processes, flows, stores, and terminators.

2. Number the processes.3. Redraw the DFD as many times as necessary for esthetics.4. Avoid overly complex DFDs.5. Make sure the DFD is internally consistent and consistent

with any associated DFDs (ie Balanced).

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Preparing Data Flow Diagrams

• Read narrative carefully - number lines and paragraphs in text

• Prepare table of entities and activities• Draw context diagram - use DFD

guidelines

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Steps for Creating E&A Table

1. Read Narrative2. Identify all Verb phrases (circle them)3. Identify all Noun phrases (rectangles or data store)4. Line by line insert noun as Entity and Verb as Activity

along with data flow noun if present5. Identify External Entities6. Identify Internal Entities

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Entities and Activities Table

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DFD Guidelines

• 1. Include on the systems documentation all (and only) activities and entities described in the systems narrative - no more, no less.

• 2. When multiple entities operate identically, depict only one to represent all.

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DFD Guidelines (Level-0 and higher)

• 5. Group activities if they occur in the same place and at the same time.

• 6. Group activities if they occur at the same time but in different places.

• 7. Group activities that seem to be logically related.• 8. To make the DFD readable, use between five and

seven bubbles.• 9. On a logical DFD, data flows cannot go from higher-

to lower-numbered bubbles.

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Preparing Systems Flowcharts

• 1. Divide the flowchart into columns; one column for each internal entity and one for each external entity. Label each column.

• 2. Flowchart columns should be laid out so that the flowchart activities flow from left to right, but you should locate columns so as to minimize crossed lines and connectors.

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System flowcharting guidelines (cont.)

• 3. Flowchart logic should flow from top to bottom and from left to right. For clarity, put arrows on all flow lines.

• 4. Keep the flowchart on one page. If you can’t, use multiple pages and connect the pages with off-page connectors. Computerized flowcharting packages will print your flowcharts only on paper that will fit in your printer!

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System flowcharting guidelines (cont.)

• 5. Within each column, there must be at least one manual process, keying operation, or data store between documents. That is, do not directly connect documents within the same column.

• 6. When crossing organizational lines (one column to another), show a document at both ends of the flow line unless the connection is so short that the intent is unambiguous.

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System flowcharting guidelines (cont.)

• 7. Documents or reports printed in a computer facility should be shown in that facility’s column first. You can then show the document or report going to the destination unit.

• 8. Documents or reports printed by a centralized computer facility on equipment located in another organizational unit ( warehouse, shipping dept.) should not be shown within the computer facility.

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System flowcharting guidelines (cont.)

• 9. Processing within an organizational unit on devices such as a PC or computerized cash register should be shown within the unit or as a separate column next to that unit, but not in the central computer facility column.

• 10. Sequential processing steps (computerized or manual) with no delay between them (and resulting from the same input) can be shown as one process or as a sequence of processes.

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System flowcharting guidelines (cont.)

• 11. The only way into a computer data storage unit is through a computer processing rectangle.

• 12. A manual process is not needed to show the sending of a document. The sending of the document should be apparent from the movement of the document itself.

• 13. Do not use a manual process to file a document. Just show the document going in to a file.