The project management triangle is often used to illustrate that project management success is measured by the project team’s ability to manage the project or part of the project so that the expected results are produced while managing time and cost.
The triple constraint is depicted as a triangle with cost, scope, and schedule as the sides of the triangle. It could be said that they contain customer satisfaction that could be considered figuratively to be the interior of the triangle since the customer should always be concerned about scope, time, and cost as well. Thus, in order to create customer satisfaction, we must perform all of the scope that was promised for the budget that we promised and deliver it when it was promised.
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The success of the project depends on the project team’s ability to control the available resources of the project in terms of time, cost, and performance. The resources that most projects need to control are money, manpower, equipment, facilities, materials, and information.
In organizations using project management, the projects begin and end, and the project team members come and go. The resources in an organization like this must be someone’s responsibility when they are not assigned to a project. These persons are the functional managers. The resources really have two managers: the project managers to whom they report when they are assigned to a project and the functional managers to whom they report when they are not assigned to a project.
The problem this creates for the project manager is that none of the people on the project team are assigned permanently to the project manager or the project. If members of the project team are dissatisfied about the progress of the project, there is a good chance they can leave the project and take another assignment. In a sense, nothing is going to get done on the project without the cooperation of the functional managers. If things go wrong on the project and the project manager needs additional resources, it will be the functional manager who juggles schedules for the resources to get the work done.
Project managers must be made responsible for the cost, schedule, and performance of the project. Many times the project manager is much more motivated to achieve high performance at the expense of cost and schedule. This is why it is important that the project manager set his or her own schedules and budgets.
This balance of the legs of a triangle is important to remember. In other words, if we or the customer want to change one of the legs, there will likely be an effect on the others. If the customer wants to shorten the schedule, we are likely to have an increase in cost or a reduction in the deliverables or both. If we add work to the project, called "scope creep," we are likely to have to increase the cost or revise the schedule or both. If the sponsor of the project wants to reduce the cost of the project, we will probably have to reduce the deliverables.
Today the customer and the stakeholders want all three and put the burden of meeting these constraints on the project manager. It then becomes the responsibility of the project manager and the project team to balance the trade-offs.