Ian Alexander is an independent consultant specialising in Requirements Engineering. He has written several training courses on systems and requirements engineering. He has led hundreds of training courses on systems engineering, requirements, DOORS, and DXL, and has run numerous practical workshops on scenarios, trade-offs and requirements. He was co-author of an Addison-Wesley book on HTML 3 and its 2nd Edition on HTML 4. He is the author of the Scenario Plus for Use Cases toolkit, and is a well-known speaker and writer on scenario usage. He is currently on a technology project to investigate the reuse of specifications for control systems in the German automobile industry. He helps to run the BCS Requirements Engineering Specialist Group and the IEE Professional Network for Systems Engineering. He is a Chartered Engineer.

Richard Stevens is the founder of QSS, the firm that launched the pioneering Requirements Management tool DOORS, the world's most popular requirements tool. He is the co-author of books on "Systems Engineering", "Software Engineering Standards", "Software Engineering Guidelines" and "Understanding Computers". In 1998, Richard was appointed as the first European Fellow of INCOSE, the International Council on Systems Engineering.

• Table of Contents

1. Introduction 9

1.1 Why do requirements matter? 9

1.2 Who are requirements for? 12

1.3 Different names for requirements 13

1.4 Different types of specification 14

1.5 The challenge of writing better requirements 15

1.6 The requirements writing process 18

2. Identifying the stakeholders 21

2.1 Different types of stakeholder 21

2.2 Your house extension: a simple case? 22

2.3 A practical approach to identifying stakeholders 23

Exercise 1: Listing the stakeholders 23

3. Gathering requirements from stakeholders 26

3.1 Possible techniques 26

Exercise 2: Asking 'why?' 28

3.2 Interviews 28

3.3 Workshops 32

3.4 Experiencing life as a user 36

3.5 Observing users at work 36

3.6 Acting out what needs to happen 36

3.7 Prototypes 38

4. Other sources of requirements 40

4.1 Possible sources 40

Exercise 3: Extracting requirements from source documents 44

Exercise 4: Extracting requirements from a memo 45

4.2 Getting requirements for mass-market products 45

4.3 User requirements in subsystem projects 46

5. Structuring the requirements 47

5.1 You need structure as well as text 47

5.2 Breaking the problem down into steps 48

5.3 Organizing requirements into scenarios 50

5.4 Examples of goal decomposition 52

Exercise 5: A structure for user requirements 53

5.5 Handling exceptions 53

Exercise 6: Could anything go wrong here? 54

Exercise 7: Exceptions 55

5.6 Examples and exercises in requirement structure 57

Exercise 8: Creating a heading structure 57

Exercise 9: The right document for each subject 57

Exercise 10: Wrongly placed requirements58

6. Requirements in context 59

6.1 The user requirements document 59

6.2 Organizing the constraints 60

Exercise 11: Writing constraints 64

6.3 Defining the scope 64

Exercise 12: Restricting the scope 65

6.4 Requirement attributes 65

6.5 Keeping track of the requirements 67

7. Requirements writing 70

7.1 Quality, not perfection 70

7.2 Sketch, then improve 70

7.3 Anatomy of a good requirement 70

7.4 Guidelines for good requirements 71

7.5 Don't write like this 72

Exercise 13: Good requirements 75

Exercise 14: Writing requirements for familiar domestic systems 75

Exercise 15: Ambiguous requirements 76

8. Checking and reviewing 78

8.1 Checking the document structure with users 78

8.2 Checking the requirements 80

Exercise 16: Checking individual requirements 81

Exercise 17: Checking a set of requirements 82

8.3 Reviewing 83

8.4 Success - the reviewed document 85

Exercise 18: Reviewing 85

A: Answers to exercises 87

Exercise 1: Listing the stakeholders 87

Exercise 2: Asking 'why?' 87

Exercise 3: Extracting requirements from source documents 87

Exercise 4: Extracting requirements from a memo 88

Exercise 5: A structure for user requirements 88

Exercise 6: Could anything go wrong here? 89

Exercise 7: Exceptions 89

Exercise 8: Creating a heading structure 90

Exercise 9: The right document for each subject 90

Exercise 10: Wrongly placed requirements 90

Exercise 11: Writing constraints 91

Exercise 12: Restricting the scope 92

Exercise 13: Good requirements 92

Exercise 14: Writing requirements for familiar domestic systems 93

Exercise 15: Ambiguous requirements 93

Exercise 16: Checking individual re