Drawing DFDs – an Editorial
When you are assigned to draw a DFD it is not an assignment about pictures. The
assignment means that you are to clearly understand the information system described
and to display that understanding in the form of a Data Flow Diagram.
The technique of figuring out how to draw a DFD can be a way of achieving the
understanding that you need.
The following points deal with the mechanics of drawing Data Flow Diagrams. They
require no understanding of the system. As such, it is an absolute requirement that
every student will conform to them in every diagram.
1) DFDs use only four symbols – you may not invent more! These are symbols for:
Externals, Processes, Data Stores, and Data Flows. You must use the shape
conventions in the text. Your symbols must be easily distinguishable and consistently
used.
2) Every symbol must have text which describes it. Here are some general rules:
a)Externals are named with singular nouns (e.g. Customer, Accounting
Department)
b) Process names generally begin with action verbs (e.g. Verify User Status,
Create Purchase Order)
c)Data Stores indicate the nature of the records which they store (e.g. Conference
Registration, Suppliers)
Note 1: Some authors consider the use of words like "file" or "record" in
data store names to be redundant since the shape tells you it is a file.
Note 2: I favor using the system owner's file names if possible even if they
are not optimum.
d) Data Flows should indicate what data is flowing (e.g. customer record,
payment.) When we use the name of a document, such as "paycheck" or "receipt"
we are understood to be talking about the data written on the document – not the
physical piece of paper. It is a common error to use verbs to name data flows
(send, give, file) Do Not do this.
3) Every data flow must be mediated by a process. That is, for every data flow symbol
(arrow) there must be a process on one or both ends. The text illustrates this principle by
showing common errors. This means:
 The only thing that an external can connect to is a process.
 The only thing that a data store can connect to is a process.
 A process may connect to a data store, an external, or another process.
4) Every process must have data flowing in AND data flowing out. Remember, between
any two boxes there is never more than one data arrow flowing in and one data arrow
flowing out.
5) Externals define the limits of the system and are considered to lie outside the system.
This means that any process performed by an external does not appear on the data flow
diagram. Note: It is common for students to confuse externals, which are sources of
data with individuals or groups which perform processes. Those individuals or groups
may provide knowledge of how to do the process, but this is not the same as data. For
example; if a process is described as "Inventory clerk uses the approved packing list to
update the inventory record." the clerk is NOT an external because they are not a source
of data.