Elementary Process Descriptions: Specifying Processes

1
Elementary Process DescriptionsSpecifying
Processes

• COMM1B
• Information Systems Analysis

2
Outline

• The need for process specification
• Bottom level processes on Process Model (DFD)
• When complete, precise, unambiguous descriptions
  are needed to define a process
• Methods
• Structured English
• Decision tables
• Decision trees
• Mathematical formulae

3
Example Specificationfor a Process

• A product is passed as fit for sale if it passes
  a mechanical test and an electrical test and has
  the correct dimensions. If it fails the
  mechanical test or the electrical test (but not
  both), it is sent back to the workshop for
  repair. In all other cases the product is
  rejected.

4
Problem with English Narrative

• Is the condition correct dimensions implied in
  the second and third paragraphs?
• Unclear - ambiguous- may or may not be

5
Using Diagrams and Tables

• When a process simply involves a number of
  decisions to be made to determine an outcome,
  then
• Decision Tree diagrams
• or
• Decision Tables
• can be used

6
Decision Tree
7
Decision Treesimplified
8
Limited Entry Decision Tables
Conditions (questions)
Condition Entries (answers)
Actions (list)
Action Entries (outcomes)
9
Limited Entry Decision Tables
10
Decision Table -Eliminating Redundancy
Correct dimensions?
Y Y Y Y N
Passed mechanical test?
Y Y N N -
Passed electrical test?
Y N Y N -
Accept product
X
Repair product
X X
Reject product
X X
11
Decision Table -Eliminating Redundancy Further
Correct dimensions?
Y Y Y
E L S E
Passed mechanical test?
Y Y N
Passed electrical test?
Y N Y
Accept product
X
Repair product
X X
Reject product
X
12
Using Structured English

• When a process is more complex and is expected to
  be computerised, Structured English is usually
  written (similar to programming code)
• Components of Structured English (logic
  constructs)
• Sequence Events happening one after another
• Selection Choices have to be made
• Iteration Repetition of an action

13
Sequence

• Multiply Price by Quantity_Sold giving Net_Price
• Multiply Net_Price by VAT_rate giving VAT_price
• Add VAT_price to Net_Price giving Gross_Price

14
Example Specification for a Process

• A product is passed as fit for sale if it passes
  a mechanical test and an electrical test and has
  the correct dimensions.
• If it fails the mechanical test or the electrical
  test (but not both), it is sent back to the
  workshop for repair.
• In all other cases the product is rejected.

15
Selection (helpful style with brackets)

• IF dimensions not OK
• Reject product
• ELSE (dimensions OK)
• IF mechanical test OK
• IF electrical test OK
• Pass product
• ELSE (electrical test not OK)
• Repair product
• ELSE (mechanical test not OK)
• IF electrical test OK
• Repair product
• ELSE (electrical test not OK)
• Reject product

16
Selection

• IF dimensions not OK
• Reject product
• ELSE
• IF mechanical test OK
• IF electrical test OK
• Pass product
• ELSE
• Repair product
• ELSE
• IF electrical test OK
• Repair product
• ELSE
• Reject product

17
Selection (formal style - technical)

• IF dimensions not OK
• Reject product
• ELSE
• IF mechanical test OK
• IF electrical test OK
• Pass product
• ELSE
• Repair product
• ENDIF
• ELSE mechanical test not OK
• IF electrical test OK
• Repair product
• ELSE electrical test not OK
• Reject product
• ENDIF
• ENDIF
• ENDIF

18
Report-like style (accessible)

• 1. If the dimensions are incorrect reject
  product
• 2. If a product has the correct dimensions,
  consider the mechanical and electrical tests as
  follows
• 2.1 Product passes both mechanical and
  electrical tests pass product
• 2.2 Product fails both mechanical and
  electrical tests reject product
• 2.3 Either mechanical test or electrical test
  failed (but not both) repair product

19
Selection using CASE structure

• Select CASE
• CASE l (O lt Net_Price lt lOO)
• Set Discount to O
• CASE 2 (100 lt Net_Price lt 200)
• Set Discount to 10
• CASE 3 (200 lt Net_Pricelt 300)
• Set Discount to 15
• CASE 4 (300 lt Net_Price lt 500)
• Set Discount to 20
• CASE 5 (Net_Price gt 500)
• Set Discount to 25

20
Iteration

• REPEAT
• Add Invoice_Total to Overall_Total
• Add 1 to No_Invoices
• UNTIL no more invoices
• Divide Overall_Total by No_Invoices giving
  Average_Value

21
Iteration

• WHILE more invoices
• Add Invoice_Total to Overall_Total
• Add 1 to No_Invoices
• ENDWHILE
• Divide Overall_Total by No_Invoices giving
  Average_Value

22
Summary

• The three main methods for specifying processes
  at their lowest level of detail (known as
  elementary process descriptions) are Decision
  trees, Decision tables and Structured English
• Sometimes a mixture of these is used to describe
  a single process
• In your assignment
• Marks for complexity, logical correctness

23
Embedded Decision Table example

• REPEAT
• READ next order
• IF regular order customer
• No discount, offer free delivery
• ELSE
• Determine discount according to following table
• ENDIF
• UNTIL no more orders

24
Further Reading

• Lejk and Deeks
• An Introduction to Systems Analysis Techniques
  2nd Edition
• Chapter 6 Specifying Processes
• S. Skidmore
• Introducing Systems Analysis
• Chapter 7
• NCC Blackwell, Oxford 1994