Data Flow Diagram(DFD)

Data Flow Diagram

Shows the flow of data through the system and the processing performed by the system.

Gane and Sarson notation is widely popular

Process

A Process is work performed by a system In response to incoming data flows or conditions and it transforms incoming data flow into outgoing data flow.

External Entity

An external Entity is an outside person (e.g. supplier, customer), organization unit (e.g. other dept), system (other business systems), or organization (e.g. Bank) that interact with the system.

Data Store

A Data Store is an “inventory”of data. That is, stored data intended for later use (data at rest). 

Data Flow

Represent inputs or outputs, to or from the processes. The arrow head indicates the direction of data flow. Label the arrows with the name of the data that moves through it.

Steps of DFD

Draw Context Diagram

Level 0 (Top Level) Data Flow Diagram

Level 1 Data Flow Diagram                                                                               

Continue up to elementary functions

Draw Context Level Diagram

Ø  Think the system as a container

Ø  Distinguish the inside from the outside

Ø  Ignore the inner workings of the container

Ø  Find out the net inputs to the system

o   Business transactions a system must respond to

Ø  For each net input determine its source (External Agents)

Ø  Find out the net outputs from the system

o   Responses produced by the system

Ø  For each net output find the destination (External Agents)

Ø  Identify any external data stores,

Files or databases of other systems

Steps for draw Context level Diagram

Ø  Identify all sources and recipients of data to/from the system.

Ø  Identify major data flows to and from the System

Ø  Convert each source or recipient into external entity

Ø  Add the data flows between each external entity and the process representing the entire system.

E.g.

Bank Payment System

Whenever an account holder wants to withdraw some cash, he pesents a cheque or withdrawal slip.

The account is checked for the appropriate balance.

If balance exists, the cash is paid and the account is updated.

Context diagram

Decomposition

Is the act of breaking a system into its component subsystems, processes and sub processes.

Steps for draw Data Flow Diagram

Step1: Decompose the system/sub system into sub process.

Step 2: Identify External Entities communicate with sub processes

Step3: Identify Data stores & Data Flows

Level 0 DFD

Fact Finding Techniques

To study any system the analyst needs to do collect facts and all relevant information. The facts when expressed in quantitative form are termed as data. The success of any project is depended upon the accuracy of available data. Accurate information can be collected with help of certain methods/ techniques. These specific methods for finding information of the system are termed as fact finding techniques. Interview, Questionnaire, Record View and Observations are the different fact finding techniques used by the analyst. The analyst may use more than one technique for investigation.

Interview
This method is used to collect the information from groups or individuals. Analyst selects the people who are related with the system for the interview. In this method the analyst sits face to face with the people and records their responses. The interviewer must plan in advance the type of questions he/ she is going to ask and should be ready to answer any type of question. He should also choose a suitable place and time which will be comfortable for the respondent.

The information collected is quite accurate and reliable as the interviewer can clear and cross check the doubts there itself. This method also helps gap the areas of misunderstandings and help to discuss about the future problems. Structured and unstructured are the two sub categories of Interview. Structured interview is more formal interview where fixed questions are asked and specific information is collected whereas unstructured interview is more or less like a casual conversation where in-depth areas topics are covered and other information apart from the topic may also be obtained.

There are two types of interviews unstructured and structured.

 Open-ended questions allow the interviewee to respond in any way that seems appropriate.

 Closed-ended questions restrict answers to either specific choices or short, direct responses.

Questionnaire
It is the technique used to extract information from number of people. This method can be adopted and used only by an skillful analyst. The Questionnaire consists of series of questions framed together in logical manner. The questions are simple, clear and to the point. This method is very useful for attaining information from people who are concerned with the usage of the system and who are living in different countries. The questionnaire can be mailed or send to people by post. This is the cheapest source of fact finding.


Record View
The information related to the system is published in the sources like newspapers, magazines, journals, documents etc. This record review helps the analyst to get valuable information about the system and the organization.

Observation
Unlike the other fact finding techniques, in this method the analyst himself visits the organization and observes and understand the flow of documents, working of the existing system, the users of the system etc. For this method to be adopted it takes an analyst to perform this job as he knows which points should be noticed and highlighted. In analyst may observe the unwanted things as well and simply cause delay in the development of the new system.