Note on Concept of System Design Tools

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Concept of System Design Tools (Application Modeling Techniques)

System analysis can be complex and confusing work. The analyst should be able to deal with a large amount of highly detailed and often conflicting information. The analyst needs a way to organize the information, determine where there are gaps in understanding and identify areas of conflicting. Modeling techniques provide solutions for the system analyst.

The modeling techniques used in system development are:

  • Context diagram
  • Data Flow Diagram (DFD)
  • E-R diagrams

Context Diagram

In keeping with the top-down approach to requirement determination, the first graphic that is produced using structured technique is the context diagram. It gives a broad overview of the information system environment including the data flows into and out of the system.

Context Diagrams serve three important purposes

  1. Context diagrams support a data-oriented approach to system design.
  2. Context diagrams help to investigate the output and process requirements of the organization.
  3. Context diagrams defines the boundaries of the proposed system.

Levels of Context Diagram

There are three levels of context diagrams:

  1. Level 1: A user-level diagram describes one functional area’s operational activity.
  2. Level 2: A combined user-level diagram provides an overall view of the activities of related user groups.
  3. Level 3: An organizational level diagram reflects a consolidated view of the activities of the organization.

Context diagram for order processing

Fig: Data flow diagram of part of an order processing system


User level contexts diagram of purchase


Fig:contexts diagram of purchase process


Combined User-level context diagram for order tracking system

Fig: Combined User-level context diagram for order tracking system


Problem Narrative

The customer sends a list of items required, which is processed by the customer handling department. A copy of the list is sent to the stores. Based on the item price, an estimated value of goods is prepared and sent to the client. At the end of the month, consolidated list of the customer requests are prepared and sent to the manager of the sales department.

Draw the context diagram based on above narrative.

Context Diagram for Retail Store

Fig: Context Diagram for Retail Store


Context Diagram for Student Registration System

Fig: Context Diagram for student Registration System (

Data Flow Diagram

A Data Flow Diagram (DFD) is a pictorial representation of the path which data takes from its initial interaction with the system until it completes any interaction. The diagram will describe the logical data flows without detailing the movements of any physical items.

The DFD also gives insight into the data that is used in the system. It does not show a sequence of steps. It shows only what the different processes in a system are and what data flows between them. Preparing context diagram is a preliminary step in creating a data flow diagram (DFD).

Based on context diagram, data flow diagrams identify the major data flows within the system boundaries, the process and the data storage.

Levels of Data Flow Diagram

The complexity of business system means that it is impossible to represent the operations of any system by means of single data flow diagram. At the top level, an overview of the different systems in any organization is shown by way of context analysis diagram. When exploded into DFD, they are represented by:

Level 0: System Input and output

Level 1: Subsystem level data flow – Functional Level

Level 2: File level detail data flow

The input and output data were shown should be consistent from one level to the next.

Level 0: System Input/output: A level-0 DFD describes the system-wide boundaries detailing inputs to and outputs from the system and major processes. This diagram is similar to the combined user-level context diagram.

Level 1: Subsystem level data flow: A level-1 DFD describes the next level of detail within the system detailing the data flows between subsystems, which make up the whole.

Level 2: File level detail data flow: A level-2 DFD details the files to which the data is applied in the system and from which data is obtained. Each individual process is shown in detail.

Order Tracking System

Level 0

Level 1

Level 2

Entity Relationship Diagrams (E-R Diagram)

Entity Relationship Diagrams (ERD) are graphic illustration used to display object or events within a system and their relationships to one another. E-R diagrams model data is much the same way as DFD's model processes and data flows.

Purpose of ER Diagram

  1. Verify accuracy and thoroughness of data design, current and new, with users.
  2. Organize and record organizational data entities, relationships and scope through decomposition and layering.
  3. Enhance the overall communication between development project team member’s system technicians, management and users with the use of graphic models.
  4. Generally, simplify and bolster the creative data design process.

Decision Tree

Decision Tree provide a graphic representation of decision logic that helps non-computer people easy to understand. The principles for the development of decision tree are relatively forward.

  1. Identify all conditions
  2. Find out values these conditions may take or assume
  3. List all possible outcomes

Decision Tree are graphical representations of the decision table. These are also available and aid the construction of decision tables.

A decision tree helps to show the paths that are possible in a decision following an action or decision by the user. A decision tree helps prefer the easier-to-follow mapping of a complex design. This mapping should show branch point forks, but not the details of the user dialogue.

Example:Decision trees help designer visualize how the user will move through the design to reach the desired location.

Decision Tables

Using decision tables, decision trees conditions and outcomes are listed in the form of two-dimensional tables. A decision table, as compared to a decision tree, checks all the possible combinations that might arise for all conditions.

General Format of Decision Tables

List of conditions

Columns representing logical combinations of conditional value

Lit of outcomes

Resulting outcome for each set of conditions

Decision table is a tabular method for describing the logic of the decisions to be taken. Decision table accompanies the flowcharts defining the possible contingencies that may be considered with the program and appropriate course of action for each contingency.

Decision tables can be divided into four parts:

1. Condition Stub:
It consists of a list of all the conditions that are to be taken account of. Condition entries that complete the condition statements. They are tabular representation of the combination of the conditions that are to be satisfied, for each of particular action, that is given a “Y” of “N” or ___x____’ mark is placed to indicate whether a particular condition is to be considered or ignored.

2. Action Stub:
It consists of all the possible actions that are to be taken.

3. Action Entries:
Action entries are entries that complete the action statements.

4. Data Dictionaries:
Data dictionary defines each term called data element, encountered during the analysis and design of the new system. Data elements can describe files, data flows or processes. For example, you want to print the vendor’s name and address at the bottom of the cheque. The dictionary might define vendor’s name and address as flows:

Vendor_name +

Street +

City +

State +

Phone +

Fax +


This identification becomes a part of the data dictionary that ultimately will list all key terms used to describe various data flows and files.

Major Symbols:

The symbols used white preparing data dictionaries are:

= equivalent to

+ and

[] either/or

() optional entry


Khanal, R.C. Khanal, R.C. Computer Concept for XII. Pashupatigriha Marga, Thapathali, Kathmandu, Nepal: Ekta Books Distributors Pvt. Ltd., 2010. 21-30.

Gurung,Juddha bhadur et.,al Computer Science-XII, Bhundripuran Pakashan,Ktm

  1. Most common System design tools are Context Diagrams, E-R Diagrams and Flow Chart.
  2. Context Diagrams is the first graphic that is produced using structured technique is the context diagram.
  3. Flow Chart is also known as Data Flow Diagram which means a pictorial representation of the path which data takes from its initial interaction with the system until it completes any interaction.
  4. E-R Diagram is a graphic illustration used to display object or events within a system and their relationships to one another.

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