All projects are carried out under certain constraints. It doesn’t matter what the project involves – it will be limited by three fundamental project constraints – scope, cost and time.
In project management jargon, this set of constraints is known by several names – the project triangle, the iron triangle, the scope-cost-time equilibrium, and the triple constraint.
If you visualise a triangle – cost, time and scope form the sides of the triangle – with quality inside the triangle as the central theme.
Quality affects, and is affected by, every side of the triangle. In the project triangle model, quality results from what you do with cost, time and scope. The initial triangle illustrates the concept that when the original cost, time and scope constraints are met, the specified project quality will be achieved.
And how are cost, time and scope defined?
- Scope. A project’s scope is its ultimate overall goal.
- Cost. All projects cost money. The project manager should have a realistic estimate of the project’s total cost before the project starts.
- Time. Individual tasks, their durations and sequencing determine how long the overall project will take to deliver and which tasks are on the critical path.
Each of these project constraints is linked to the other two. If one or more of the constraints is changed, the remaining ones will also be changed. For instance, decreasing the budget of a project is likely to lengthen its schedule or force the creation of a new, more restrained scope. Or an increase in scope generally results in an increase in cost and time. Likewise, an acceleration of the schedule may produce an increase in costs and a reduction in scope.
The project management challenge is to balance these constraints to create the best possible scope-cost-time equilibrium. The project manager has to be constantly aware of this equilibrium and trade-off between these constraints while ensuring the project stays on track for its original goals.
In reality, project constraints are more than a triangle, because other factors have to be dealt with as well. For example, resources can be a significant project constraint. Although everyone may agree on scope, cost and time, a resource shortage could require a longer schedule or a higher cost to employ additional workers. Even though the estimated cost may be acceptable, the schedule requires people, equipment, or materials that are not readily available. You could afford them, but there are none in the marketplace. This has contributed to the development of other constraint models such as the project diamond and the six pointed star.
However, the project triangle is still valid, particularly because it provides a useful graphical tool for communicating about the flow-on effects of changes on a project. Due to this, the project triangle will remain an important component in the project manager’s toolkit.
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