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The Development Project: Analysis and Design

Data flow diagrams at level 0 and level 1

  • 3 things to know
  • A2 Unit 2

Overview

Data flow diagram, balancing between levels and context diagram for a cinema booking system.

What you need to know

Answer each one before you reveal it.

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Key term

What is a data flow diagram?

Key term

Name the four symbols used on a data flow diagram and what each represents.

Process

Why must every process have both an input and an output?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is a data flow diagram?

    Model answer

    A data flow diagram shows how data moves through a system: where it comes from, what processes act on it, where it is stored, and where it goes. It shows the flow of data and deliberately says nothing about timing or sequence.

  2. 2

    Name the four symbols used on a data flow diagram and what each represents.

    Model answer

    An external entity is a source or destination of data outside the system; a process transforms data; a data store holds data; and an arrow is a data flow, labelled with the data it carries.

  3. 3

    What is a level 0 data flow diagram?

    Model answer

    A level 0 diagram, also called a context diagram, shows the whole system as a single process with the external entities around it and the data flowing between them. It fixes the boundary of the system and nothing else.

  4. 4

    Why is the context diagram drawn first?

    Model answer

    Drawing it forces agreement on what is inside the system and what is outside before any internal detail is designed, which is the decision that costs most to change later.

  5. 5

    What is a level 1 data flow diagram?

    Model answer

    A level 1 diagram opens up the single process of the context diagram into the main processes of the system, showing the data stores and the flows between those processes, while keeping the same external entities and the same flows across the boundary.

  6. 6

    What is meant by balancing between levels?

    Model answer

    Balancing means the data flowing into and out of the system on the level 1 diagram must be exactly the flows shown on the context diagram. A flow that appears at one level and not the other is an error in the model.

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