Triple Constraints Model

The triple constraints model has been one of the main staples for teaching project management for as long as I can remember. The model is generally represented by a triangle with Scope on the horizontal leg, Time on the left leg, Cost or Resources on the right leg and Quality in the center of the triangle.

There are variations of the triple constraints theme and name, but the concept is generally the same. The idea has to do with priorities in a project. The stakeholder must decide, in regards to a particular deliverable, which of the three constraints of scope, time and cost is most flexible, medium flexible, and least flexible. Let's imagine a stakeholder who decides that, for a particular deliverable, time is the most flexible, scope is medium flexible and cost is least flexible. This would mean that, if need be, they would prefer a deliverable be later than originally planned as opposed to the deliverable costing more or having fewer features.

The rule is: if one of the triple constraints is changed, it will most likely affect one or both of the other constraints in addition to affecting the quality of the deliverable. An effective way to demonstrate this concept is to say to the stakeholder, "I have three cards: better, faster, and cheaper. You can only have two cards. Which of the two cards do you want?"

Upon reviewing A Guide to the Project Management Body of Knowledge (PMBOK® Guide)- 4th Edition, which came out in December, 2008, and will be the basis of the Project Management Professional (PMP®) certification test beginning June 30th, 2009, I did a search for the term "triple constraints model" and it was nowhere to be found. In fact, I couldn’t find the word "triple" in the PMBOK® Guide. The closest reference I could find in the PMBOK® Guide - 4th Edition to the triple constraints concept was under Risk -  section 11.3, page 289. Although the triple constraints model may not be in the PMBOK® Guide, I believe it to be a Best Practice and a great tool for eliciting requirements and managing change control.

The PMBOK® Guide does have many references to constraints, and rightfully so. Constraints are a major consideration in project planning and execution. Theory of Constraints by Dr. Eliyahu M. Goldratt is an excellent book for exploring the impact of constraints. I just find it odd that the PMI® has left out this valuable model for teaching.  What are your thoughts?

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