5.2 - Data Flow Diagrams
2016 - Unit 2
A data flow diagram is a visual representation of how data is transferred within a system or organisation. Data flow diagrams are often abbreviated to just 'DFD'.
You will need to recognise the symbols, understand the diagrams and explain how data flow can be affected.
Data Flow Diagram Symbols
The common symbols used in a DFD are shown below:
External Entity (e.g. Employee, Customer)
Alternative DFD symbols you may see on the exam paper (both sets of symbols have been accepted in previous exam papers):
Process (e.g. Calculate Pay, Review Application)
Data Store (e.g. Staff Database, Customer Database)
Level 0 data flow diagrams show the transfer of data in a simple manner. Processes may be generalised and not shown separately, for a more basic overview.
Below is an example of a Level 0 DFD to show the process of an employee submitting an application to be promoted:
Level 1 data flow diagrams show the transfer of data in a more detailed manner. Processes are shown separately, for a more complex and realistic overview.
Below is an example of a Level 1 DFD to show the process of an employee submitting an application to be promoted. The individual processes have been expanded from the Level 0 DFD above to show how the application must first be signed off by an administrator before it is reviewed by management:
Rules for Drawing a Level 1 DFD
Data flows only in one direction.
Every data flow is labelled (with the data itself, not the action).
Every data flow connects to at least one process.
At least one input or output for each external entity.
Impacts Affecting the Flow of Information
There are several reasons why the flow of information within an organisation may be delayed or inefficient, such as:
e.g. information is entered incorrectly, causing delays.
e.g. information is lost or staff don't follow protocol.
e.g. network connection breaking or system failure.
e.g. meetings postponed or emails unread.
5.2 - Data Flow Diagrams:
1. Draw the symbols for the four parts of a data flow diagram. 
2. Look at the image of the Level 1 DFD. State one example of:
An external entity
A data flow
A data store [1 each]
3. Give one difference between a Level 0 and Level 1 DFD. 
4. State three rules for drawing a correct data flow diagram. 
5. After heavy snow, a school has decided to shut for the next two days. Many parents were not informed of this decision and are angry that they brought their children in. Give three reasons why the transfer of data (the data being a message about school closure) might not have flowed efficiently in this example.