From a Triple Constraint to Six Constraints

Posted on February 20, 2009 by


roll the dice for the constraints

I'm a purist - it's still the triple constraints for me!

Central to the work of a project manager is balancing competing demands. The term “triple constraint” is a well-known phrase in project management that refers to the competing demands of scope, time, and cost. The manner in which these three demands are balanced affects quality. If one of these factors is affected, at least one other factor will also be affected.

For example, if the scope of the project increases, there will need to be an increase in the amount of time to complete the project, an increase in costs, or both. The new, fourth edition PMBOK® changes the name “time” to “schedule” and changes “cost” to “budget”, plus adds three new constraints.



The six constraints are:

      • Scope,
      • Quality,
      • Schedule,
      • Budget,
      • Resources, and
      • Risk.

Quality isn’t merely affected by how the constraints are balanced—it is considered an actual constraint. Especially when dealing with highly-skilled individuals as resources, it makes sense to consider them a unique constraint as there may be very few each with limited time to commit to a single project. If stakeholders decide to change their tolerance for risk, this will affect other constraints as well. How to balance these constraints is a skill, and how they should be balanced differs across projects. For example, a certain deadline might be crucial to meet, and the project may be given as much funds as they need in order to meet that deadline. Conversely, it may be impossible to increase the budget, so the scope and quality have to be reduced.

As a purist it still can be helpful to consider how the original triple constraint (schedule, budget and scope/requirements) impact the “newbee” constraints. For example if a project manager’s budget is cut 10% and two additional requirements are added, the impact on negative risk goes up and quality typically is reduced.  This forms an independent / dependent variable relationship.  So yes, think of all Six constraints, but reflect on the power of the original Three!

In your experience, what has been the most important constraint in your projects?