CONTEXT AND DATA FLOW DIAGRAMS
This material uses handouts from a CSTA workshop conducted on 22 October 2007 by Alex Brown.
It is a step by step process to show students how to create their context diagram and translate it into a
data flow diagram. If they learn the process, they can do it well in their exams.
The students use PowerPoint slides as their pages, particularly to take advantage of the connector lines.
They keep adding items to the page and end up with one page for the context diagram and one page for
the data flow diagram.
Notes 1
Introductory activity
Using the scenario ask students to circle any
external identities they can identify (3) and
then underline any data items (4).
Context and Data Flow Diagrams activity – Coral Shand
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(A) Create slide 1
(B) Create slide 2
(C) Naming the data flows
Complete this table showing the information given in the scenario and the data it refers to. It is
important to make sure the data flow column actually lists data (nouns).
Information in the scenario
Membership form
Planned activities
Monthly newsletter
Membership renewal
Name the data flows
(D) Match the data flows to the external entities and determine the direction of the arrows
This is a critical step so make sure students get it right. Students should use the 4 names from the table
above in the correct columns in the table below. Some cells will be blank and/or have multiple entries.
External Entity
Data flow IN
(into the system)
Data flow OUT
(out of the system)
Prospective member
Member
Planning committee
Context and Data Flow Diagrams activity – Coral Shand
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(E) Creating the context diagram.
When you add the information from the table you get your context diagram.
Make sure students save their work at this point.
(F) Identify the processes for the data flow diagram
The data flow arrows in the context diagram now become the processes for the data flow diagram as
follows. Students should complete the table. ** Processes must start with a verb**
Data flow arrows in Context Diagram
Processes for Data Flow Diagram
(F) Add the processes and external entities
Use a new slide which will become the Data Flow
Diagram.
Copy and paste external entities from the
Context Diagram to save time.
Context and Data Flow Diagrams activity – Coral Shand
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(G) Identify data stores and add them to the DFD slide
You may want to use a separate slide to create a data store image.
Add these components to your DFD diagram.
** Taking some time to think about where to place them is a
beneficial activity.
(H) Add the labelled data flows.
Make sure the data has a different name in and out of the process circle (otherwise there was no need
for a process!). The data remains the same in and out of a data store however.
Partial example:
Example of a finished
diagram:
Save work.
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(H) Students now check what they have done against these rules - and modify their diagrams if
needed.
Other scenarios you might use to practise this process (provided by Alex Brown).
The Music Society
The secretary of The Music Society has set up a system for keeping track of the society's activities and
members. To join the society, prospective members must be nominated by existing members who pass
the new member details on to the secretary. New member details are then added to the member file
and confirmation of membership letters are sent out.
The Music Society committee meets regularly to plan activities and these are added by the secretary to
the activities file. A monthly newsletter, listing all coming activities, is compiled and sent to each
member. Advertisements are also sent to the local newspaper to inform the general public of the
society's coming activities.
Video Easy
Video Easy is open from 9am to 11pm seven days a week. To borrow videos customers must first
become members by providing proof of identity and personal details. Members can borrow any number
of videos, but before doing so they must provide their membership cards and have no videos overdue.
The staff and Video Easy search the loans file for overdue videos at close of business each day. Members
with videos overdue by a week or more are sent a warning letter informing them that their borrowing
privileges may be suspended.
Video Easy Extension
Video Easy staff are continually adding to the stock of videos. They receive catalogues of latest releases
which they use to help them order new videos from the suppliers. They make payment for these new
videos when they receive the invoice that always arrives with the new stock.
Context and Data Flow Diagrams activity – Coral Shand
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