Spreadsheets in Finance and Forecasting Project Session 3b(ii) Data Flow Diagrams

Objectives After this session you will: know what is meant by a Data Flow Diagram (DFD) recognise and be able to work with DFD symbols Construct Context Diagrams and Top Level DFDs to model data flow in a system.

Dealing with Data We have already discussed methods for collecting data, and some of the sources that you might want to explore. In this session we look at what you should do with the information that you collect.

Modelling Data Flow We create models of Data Flow in order to understand and document the information on the right: There are several different conventions which can be used. Who or what initiates the data (Data source)? In what manner is it processed, and who does it? Who uses the data? In what manner and where is the data stored? What is the output, and who receives the data (Data Target)?

Data Flow Diagrams (DFDs) DFDs model events and processes within a system. Processes are activities which transform data DFDs examine how data flows into or out of a system Note that data = any “thing”, i.e.. raw materials, information, ideas etc, anything which can be processed

DFD Questions What processing is done? When is it done? How is it done? Where is it done? By whom is it done?

DFD Questions What data is needed? For what is it needed? When is it needed? By whom is it needed?

Principles Every system can be decomposed into subsystems, and those into lower subsystems and so on. Each sub system represents a process in which data is modified. Each process has the characteristics of a system, and must have input & output. Data flows into the system, flows between processes and is then output from the system.

DFD Notation Processes transform or manipulate data – “do this” Data Flows show data going in to or coming from a process flows must start or end at a process Data Stores where data is held External Entities sources or targets for the data

DFD Notation Process boxes are numbered with a unique identifier (top left) The top line gives the location of or the people responsible for the process Data Stores are also numbered

The Context Diagram The Context Diagram is an overall, simplified view of the system, which contains only one process box, and the primary inputs and outputs This example is part of a “Home Catalogue” sales system

The Top Level DVD The purpose of this is to show the major processes which are included in the system

DFD Example A Health Club keeps information on its members Membership is subdivided into Full, Daily and Associate On the 1 st of each month, the payments clerk checks the records and sends out reminders to members whose subscription expires this month. When payment is received, the subscription is renewed.

Context Diagram Members Club System for Renewing Membership 0 member details reminder The oval boxes are external to the system. They are the sources and targets of information. renewal payment details receipt and membership details In this case, the members are both sources and targets and the data flows in different directions. The diagonal lines indicate that they are actually the same thing.

Level 1 DVD: Reminders Payments Clerk D1Member Details Office Check Records 1 D2Payment Records Office Prepare Reminder 2 Member reminder name, address member ID member payment status memo on 1 st Month member ID

DFD Task What follows is a description of a situation involving the flow of data in a Theatre Booking system You should read the description and try to construct a context diagram, and a Level 1 DFD

Theatre Example A theatre has a booking system, and keeps records of customers, plays and seat bookings A customer may make a booking in person, by telephone or by a pre-printed form At the time of booking, a clerk checks to see if seats are free; if so, she reserves the seats and checks to see if the details of the customer are on the file; if they are not, she types them in. Tickets are then printed and handed or sent to the customer Payment can be in cash or by credit card.

Finally A solution to the Theatre problem will be available next week. On the resource sheet is an important link which develops the ideas of DFDs much further