The Iron Triangle Defined
Two other terms often used to refer to the iron triangle of project management, triple constraint and flexibility matrix, can help us better define this concept.
The 'constraint' in the term triple constraint refers to the basic factors that need to be accounted for during planning and delivery of any project. These three factors are the well-known attributes of scope, schedule, and cost. Scope refers to the total amount of work involved in delivering the project, cost refers to the sum of all resources required to deliver that work, and schedule reflects the time estimated or allotted to the project's delivery. And the word 'flexibility' in the term flexibility matrix indicates that these three factors can, and should be, linked together.
The basic premise of the iron triangle is that if you are required to make a change in one of these areas, at least one of the other constraint points will also need to change to balance the overall effect. This relationship is often pictured in the form of a triangle, with dynamic links between each of the constraint attributes that sit at each vertex.
The Iron Triangle in Action
We can go back to our initial example to see how this concept can be used to help resolve any issues that arise as a project moves forward. In our hypothetical case, let's assume that one of your sales staff promised something to a key client that was not in the original plan. At the time that change in scope was made, which would likely have been done through a change request, nothing was removed to account for the addition of the new feature. To avoid problems with project, it would have been necessary to change either the schedule or cost estimates to account for the alteration to the initial plan. The overall project is constrained by these three factors in such a way that changes in one area must be reflected by changes to the plan in another area, or the project will go out of bounds.
The notion of flexibility can also be applied to help guide the best way to react in these types of situations. At the start of any project, it is useful to agree on which of the three factors is the easiest to change and which is the most difficult to change. For example, a firm schedule commitment that includes associated penalties for any delayed delivery will probably mean that schedule is considered the least flexible component. If cost has been designated as the most flexible component in your delivery, you might move to keep the project on schedule by working extra hours that incur an additional resource cost. Or if the overall scope is actually the most flexible constraint, you would first look to remove some of the lowest priority scope items in order to keep the schedule in bounds.
Other Constraint Considerations
The constraints discussed in this model are typically measured against an overall project plan, and adjusted accordingly. For example, budget is a constraint term that is often used in place of cost, since this term more accurately reflects the planned costs that were used to form the project baseline. In the same vein, personnel resources often make up the bulk of a project's total cost structure. When combined with the fact that personnel resources may need to be shared across projects, then using resource as one of the key constraint points may be more beneficial in understanding the relationship between key constraint factors.
Similarly, quality and risk can be added to the discussion as additional constraints. Some organizations prefer to consider quality as the logical outcome of adhering to the actual triple constraint principles, and assume that poor quality may result if any of the constraints are ignored. In this case quality is often placed in the middle of the iron triangle as a standard focal point throughout the delivery. But some projects may have valid quality objectives that differ from other projects, in which case adding quality to the constraint matrix to form a diamond might be a better way to proceed.
In some project management practices, risk is viewed as a totally separate factor that requires its own set of control mechanisms. However, it can also be managed within the overall framework of the flexibility matrix to add yet another point of associated concern. If any additional factors are included in the discussion, the key concept remains the same. In each case, making changes in one of the agreed to constraints should be reflected by making appropriate changes elsewhere in the constraint model.
Lesson Summary
The iron triangle, also referred to as the triple constraint or flexibility matrix, is a way to reconcile the key factors of scope, schedule, and cost as competing constraints on any project. Whenever a change is made that affects one area, a corresponding change will need to be made in another area in order to keep the project within the constraint boundaries. Budget is often used in place of cost in this model, and resource is often used as a constraint in the same manner. Quality can be viewed as an outcome of adhering to the triple constraint, or can be included as an additional constraint point. Similarly, risk is often considered as part of the overall flexibility matrix.
