There are not many books available that cover the topic of when to apply an agile or plan driven method. In my opinion the selection of when to apply a certain technique or process component remains the most difficult area of Project Management. I only know of two books that cover this in detail, my own and "Balancing Agility And Discipline" by Barry Boehm and Richard Turner.

First of all, the fact that the book is written by Boehm and Turner is remarkable. Boehm wrote the classic "Software Engineering Economics", formulated Theory-W, invented the spiral model, a true influential pioneer. Turner helped shape the CMMI model. So, both respected "old school" software engineering professionals. The fact that they try to have an impartial look at agile is an accomplishment on its own.

The book provides insights in when to apply agile methods and when to apply plan-driven methods. The authors argue that for every type there is a kind of project that is suited for one of the approaches. Or a kind of combination.

Five factors are considered influential when considering an approach:

• Critically: if the end result doesn't work, are people going to die or is it a minor inconvenience?
• Personnel: how good are your people?
• Dynamism: are requirements changing all over the place?
• Culture: do people like change or do they need order?
• Size: how many people are involved in the project?

Presented is a risk-driven approach to select the proper method. Some general project environment risks (E) are considered, risks associated with the use of agile methods (A) and risks that occur when plan-driven methods (P) are used. An assessment of these risks should help you to make a proper choice of method.

The risks are:

• E-Tech: Technology uncertainties.
• E-Coord: A lot of stakeholders to coordinate.
• E-Complex: The complexity of the system.
• A-Scale: Scalability and critically.
• A-YAGNI: Use of simple design or YAGNI (You Aint Gonna Need It).
• A-Churn: Personnel turn-over and churn.
• A-Skill: Not enough people skilled in agile methods.
• P-Change: Rapid change.
• P-Speed: Need for rapid results.
• P-Emerge: Emergent requirements.
• P-Skill: Not enough people skilled in plan-driven methods.

What I find very useful is the appendix with a comparison of a lot of methods like Scrum, RUP, Crystal, CMMI etc.

Although the book is very dry, although you don't get a ready recipe for success (duh!), this book will make your mind better suited for balancing different methods.