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chaPter 8 • analyzing systems Using Data Dictionaries 213

the external entity CUSTOMER is the source and that PROCESS 1 is the destination, provid-
ing linkage back to the data flow diagram. The checked box for “Screen” indicates that the flow
represents an input screen. It could be any screen, such as a web page, graphical user interface
(GUI), mobile phone, or perhaps a mainframe screen. The detailed description of the data flow
could appear on this form, or it could be represented as a data structure.
Data flows for all inputs and outputs should be described first because they usually represent
the human interface, followed by the intermediate data flows and the data flows to and from data
stores. The detail of each data flow is described using elements, sometimes called fields; a data
structure; or a group of elements.
A simple data flow may be described using a single element, such as a customer number
used by an inquiry program to find the matching customer record.

Describing Data Structures
Data structures are usually described using algebraic notation. This method allows an analyst
to produce a view of the elements that make up the data structure, along with information about
those elements. For instance, the analyst will denote whether there are many of the same elements
in the data structure (a repeating group) or whether two elements may exist mutually exclusive of
each other. The algebraic notation uses the following symbols:
1. An equal sign (=) means “is composed of.”
2. A plus sign (+) means “and.”
3. Braces { } indicate repetitive elements, also called repeating groups or tables. There may
be one repeating element or several in a group. The repeating group may have condi-
tions, such as a fixed number of repetitions, or upper and lower limits for the number of
repetitions.
4. Brackets [ ] are used for an either/or situation. Either one element or another may be pres-
ent, but not both. The elements listed between the brackets are mutually exclusive.
5. Parentheses ( ) are used for an optional element. Optional elements may be left blank on
entry screens and may contain spaces or zeros for numeric fields in file structures.
Figure 8.4 is an example of the data structure for adding a customer order at World’s Trend
Catalog Division. Each NEW CUSTOMER screen consists of the entries found on the right side
of the equal signs. Some of the entries are elements, but others, such as CUSTOMER NAME,
ADDRESS, and TELEPHONE, are groups of elements or structural records. For example,
CUSTOMER NAME is made up of FIRST NAME, MIDDLE INITIAL, and LAST NAME.
Each structural record must be further defined until the entire set is broken down into its com-
ponent elements. Notice that following the definition for the CUSTOMER ORDER screen are
definitions for each structural record. Even a field as simple as the TELEPHONE NUMBER is
defined as a structure so that the area code may be processed individually.
Structural records and elements that are used in many different systems are given a nonsys-
tem-specific name, such as street, city, and zip, that does not reflect the functional area in which
they are used. This method allows the analyst to define these records once and use them in many
different applications. For example, a city may be a customer city, a supplier city, or an employee
city. Notice the use of parentheses to indicate that (MIDDLE INITIAL), (APARTMENT), and
(ZIP EXPANSION) are optional ORDER information (but not more than one). Indicate the OR
condition by enclosing the options in square brackets and separating them with the symbol.

Logical and Physical Data Structures
When data structures are first defined, only the data elements that the user would see, such as
a name, address, and balance due, are included. This stage is the logical design, showing what
data the business needs for its day-to-day operations. As we learned from HCI, it is important
that the logical design accurately reflect the mental model of how the user views the system.
Using the logical design as a basis, the analyst then designs the physical data structures, which
include additional elements necessary for implementing the system. The following are examples
of physical design elements:
1. Key fields used to locate records in a database table. An example is an item number, which
is not required for a business to function but is necessary for identifying and locating com-
puter records.