Mypresentation

Information Systems
Development Methods

Jackie Croft
Room N5.107
croftj@wmin.ac.uk

ISDM11 Lecture 3 Structured
Methods 1

Information Systems
Development Methods

Structured Methods

Methods 2

Pre-methodology
Problems?

Methods 3

The Systems Development
Life Cycle - Waterfall
Strategy
Planning

Feasibility

Analysis

Design

Implementation

Maintenance

Methods 4

Strategy Planning
 Formal mechanism for deciding which
areas of the business require new or
enhanced computer systems

 Involves assessing the relative priorities
of different areas, with a view to
initiating one or more development
projects.
Methods 5

Feasibility Study
 Establish the feasibility of potential
systems ideas from strategy planning
 Look at economic, technical and
operational feasibility (see Kendall and
Kendall)

Methods 6

Systems Analysis

 Establish the requirements of users,
and hence of the business
 Concentrate on what it should deliver,
rather than how it should deliver it

Methods 7

System Design

 Translate the user requirements
gathered during systems analysis into a
computer system design
 Detail exactly how the requirements will
be satisfied

Methods 8

Implementation
 The system design provides a blueprint
for building, testing and introducing the
new system
 Programs are constructed and
hardware is installed
 Provide training for users and
assistance in cutting over to the new
system
Methods 9

Maintenance
 Production or operational phase
 The period when the system is up and
running in support of the business
 The system needs to be kept up to date
in responding to changing requirements
and system errors
 Uses 70% of the total development effort
required over the life of a system
Methods 10

Structured Methods
Structured methods consist of:
1 A default structure of steps and tasks
which the project team should consider
following
2 A set of techniques to be applied in each
step that provide (largely diagrammatic)
structured definitions of user
requirements and system components.
Methods 11

Why are Structured
Methods Desirable?

 Diagrams!!!

 Improve communications - diagrams

 Reduce ambiguity - diagrams

Methods 12

Advantages of Structured
Methods
Structured methods use the following
core concepts:
 Abstraction

 Diagrammatic modelling techniques

 User involvement

Methods 13

Abstraction
 Simplifies the area under study
 Concentrates on certain aspects while
disregarding others
 Look at the physical and logical
(conceptual) levels separately
 Can then consider the organisational
aspects
Methods 14

Physical and Logical Levels of
Abstraction (Tudor and Tudor)

Existing Physical Required Physical

Required Logical
Existing Logical

User/business
Requirements
Methods 15

Physical, Logical and Organisational
levels of Abstraction (Tudor and Tudor)
Required Physical (Operational System)
Existing Physical
System
Required Organisational

Required Logical
Existing Logical (Conceptual, Essential)
(Conceptual, Essential)

User/business
Requirements
User/business
Requirements

Methods 16

Three-Schema Architecture
 Conceptual Model – required logical
data and processes
 External Design – required system
DFDs, functions, menus and dialogues
 Internal Design – physical database
and physical process design
 All contribute to the construction
Methods 17

Diagrammatic Modelling
Techniques

 Models are produced for each level of
abstraction
 Can produce diagrams to model
processes, data and events

Methods 18

Diagrams
Can model:
 Data

 Processes

 Events

 System boundary

 Physical and logical

Methods 19

The Development of
Structured Methods
 Structured programming
 Structured design
 Structured analysis – the process view
 Structured analysis – the data view
 Structured analysis – the event view
 Blended methods

Methods 20

Structured Programming
 Dijkstra (1965) suggested that the
greatest single problem was the ‘GOTO’
statement
 Bohm and Jacopini (1966) proved that
any program could be written using
three basic constructs: sequence,
selection and iteration

Methods 21

Structured Design
Advantages:
 ‘divide and conquer’ approach

 using a modular approach, several

developers can work on a project
 information hiding

 cohesion

 coupling

Methods 22

Structured Analysis
 Emphasis shifted to an earlier part of
the SDLC
 Uses DFDs, ERDs, Data Dictionary

See: DeMarco (1978)

Methods 23

Analysis Considered
 More emphasis on the systems
analysis and design phases of the
SDLC
 Roots in process flow diagrams used in
industrial engineering
 More emphasis on systems for the
organisation as a whole

Methods 24

Process Driven Methods
Problem:
 Processes are quite volatile and liable

to change

Methods 25

Modelling Processes
 Early structured methods (DeMarco,
Youdon) concentrated on modelling the
processes and the data flowing
between them
 DFDs originated with engineering flow
diagrams

Methods 26

Processes – Context Diagram
a

Supplier
b

Purchaser
Payment
Delivery Note

Purchase Order Rejected P.O.
Copy #2
Delivery Invoice
Details
Matched P.O.
Copy #2
e d
SRW
Matched Invoice Depot Despatch Note
Accounts Customer
System

Stock Report
Matched C.O. Customer Order
Copy #1

P.O.Quantities
b Customer Order c

Sales and
Purchaser Marketing

Methods 27

DFD
a b c
Sales and
Purchaser Supplier Marketing

Purchase Order

Rejected P.O.
Copy #2
P.O.Quantities Delivery
Details
Customer Order
1 P.O.Clerk
Place Product
Product Details M3
and Monitor Info
Orders

5 Despatch Clk f
Customer 2 x C.O. Copies Despatch Report
2 x Rejected M4 Allocate Despatch
P.O.Copies Orders
Despatch Supervisor
Rejected P.O.
Copy #1
2 x P.O. Copies Stock To Current
b Be Used Stock Levels
T1 Rejected
(M) P.O.'s
Supplier 2 x C.O. Copies M2 Stock
Matched
Matched Despatch Rpt
2 x Rejected Despatch Rpt
Purchase
M1
Delivery Note P.O.Copies Orders
d

2 Goods In 5 Despatch Clk
Customer
2 x P.O. Copies Despatch Note
Check Purchase Complete
M1
Delivery Orders Customer
Order Matched C.O. c
Copy #1
Sales and
Invoice Copy Marketing
Matched P.O.
Copy #1
Matched P.O. Matched C.O.
Copy #2 Copy #2

T2 Matched P.O. Copy #1
P.O. Copy #1
(M) P.O.'s

a 7 P.O.Clerk e
Product
Matched P.O. M3 Matched Invoice
Info Match
Purchaser Copy #1 Supplier Accounts
Invoice
Product Info

3 Stock Dept
Store
New Stock M2 Stock
New
Stock

Stock Report Invoice
Stock Info

Adjustment
4 Stock Clerk f b
Maintain Despatch
Despatch
Stock Rpt Copy Supplier
Supervisor
Information

Methods 28

BAM with Potential System
Boundary Receive
Customer
Order

Arrange
Despatch
Details

Outside

Forward
Despatch
Contents

Assemble
Goods
for Despatch

Methods 29

Main Methods:
Process-Driven:
 Largely superseded by data-driven and

mixed methodologies as databases
assumed a more prominent role.

 Look at work by DeMarco, Gane and
Sarson, Yourdon and Constantine
Methods 30

Data Driven Approaches
 Early methods were process-oriented
(computers were able to carry out
functions long before they were able to
implement a database)
 The seventies and eighties saw work on
relational databases (Codd, Chen etc)
(See Codd, 1970 onwards; Chen, 1976; Jackson, 1975,
1983; Martin and Finkelstein, 1981; Orr, 1977;
Warnier, 1976 etc)
Methods 31

Data Driven Approaches
 The eighties saw the rise of methods
based on data modelling

 Information Engineering

Methods 32

Modelling Data
 The structure of the data is important
and often more stable than the
processing
 E-R Modelling, (data modelling)
techniques were introduced

Methods 33

Data

Transfer Delivery Line

made of delivery of

delivered by

part of
delivered by

transfer of
Transfer Line Stock
reduced by
transfer from

Methods 34

Summary of Process-Driven
and Data-Driven Perspectives
 Most structured methods are either
process-driven or data-driven although
some have aspects of both and a few
also incorporate a time dimension
 Structured approach is better suited to
business data processing than real time
systems development

Methods 35

The Event View
 So far the behaviour or state
perspective has been ignored

 Introduction of the State Transition
Diagram to allow for a behaviour or
state perspective

Methods 36

Integrative Approaches
 Sometimes known as blended

 SSADM and JSD take a combined view
of data, process and events

 SSADM uses Entity Life Histories

Methods 37

Event Modelling
 Event modelling (and the effect of the
event on the data) became increasingly
important
 State-oriented approach (after and before
states)
 Command-oriented approach (procedural
description of the command
 Interaction-oriented approach (rules and
constraints controlling the interaction
between events, processing and data
Methods 38

Event

Purchase Order
Line

assumed case alternative case

Q3 Q1 Q4 Q2
Delivery
Confirmation
(first)

Q4 Q2 Q3
9
1,3/4 Delivery
Ad Hoc Purchase
Confirmation events
Order Raised
(subsequent)

R1 R4 R2 R3
1 2 3 4 5 6 7 8 9
Q4 Q2 Q1 Delivery
-/1 4,5/5 Purchase Order Supplier Ceased
Out of Time Confirmation
Cancellation Trading
Purchase Order Purchase Order
(last)
Proposal Confirmation
1,2,3/6 1-5/7 1-5/8 1,3,4,5

1 2 3 4 5 6 7 8

-/2 2/3

Methods 39

Another view of event
Delivery Number
Delivery Date
Supplier Number
Depot Number
Purchase Order Number Supplier Depot
Delivery Start Time
Delivery End Time
Product Number
Quantity Due

Delivery Set of
Delivery Line

Purchase Order
Delivery Line
Line

Purchase Order Line Purchase Order Line Purchase Order Line
(subsequent) (first) (last)

Methods 40

Factors Modelled by
Structured Techniques

 An event in the outside world triggers a
process. This causes and effect on
data in a given state, and may cause
the data to be transformed to a different
state.

Methods 41

Techniques
 Wide range of techniques – some
linked to a specific method, eg SSADM
 Many ways of drawing some of the
diagrams – LDS, ERD etc
 Some techniques involve description
rather than diagrams

Methods 42

More Techniques
 Think about what you are trying to model
 Requirements
 Processes
 Data
 Events
 Which diagram is clearer or more specific
 Is a particular notation mandated by your
chosen method

Methods 43

User Involvement
 Different structured methods differ in
the amount of user involvement
 Sometimes the user involvement is not
explicit

Methods 44

How Do Methods Differ?
 Life-cycle coverage
 Underlying philosophy
 User role
 ‘Structuredness’
 Size of system aimed at
 Techniques within the method
 CASE tool support
Methods 45

Poor Quality Still!!!!!
 Information system does not address
the right problem
 Wider social or psychological problems
are missed
 Information needs not recognised,
ignorance of what may be possible
 System developed for the wrong
reasons - technological push, or
political push
Methods 46

Read:
 Kendall and Kendall, Systems Analysis
and Design, latest edition
 Tudor and Tudor; Systems Analysis and
Design A Comparison of Structured
Methods, Palgrave, 1997
 Dijkstra, E. W. (March 1968). "Letters to
the editor: go to statement considered
harmful". Communications of the ACM
11 (3): 147–148
Methods 47

Read………
 Bohm and Jacopini (1966), "Flow Diagrams,
Turing Machines, and Languages with Only
Two Formation Rules," Communications of the
ACM 9:5, p. 266, May 1966.
 DeMarco, Tom. Structured Analysis and
System Specification.
 Yourdon, Gane and Sarson, Yourdon and
Constantine etc
 Codd, 1970 onwards work on relational
databases
Methods 48

Read……..
 Chen (ER Model)
 Michael Jackson, Jackson Structured
Programming, Jackson Structured
Development
 Martin, James and Clive Finkelstein. Nov
1981. "Information Engineering",
Technical Report, two volumes, Lancs,
UK : Savant Institute, Carnforth.

Methods 49

Read…..
 Kenneth T. Orr, Structured
Systems Development, Prentice
Hall PTR ;
 Weaver, Lambrou and Walkley.
Practical Business Systems
Development Using SSADM,
Prentice Hall, 3rd ed, 2002
Methods 50

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