Triple constraints of a project

The triple constraint (iron triangle) says that every project is limited by three linked factors: scope, time and cost. Quality depends on how well these three are balanced. Changing one usually forces a change in another.

              SCOPE
               /\
              /  \
             / Q  \
            /______\
          TIME     COST

The three constraints

• Scope: the work needed to deliver the product or service, with all its features and requirements. Question: what work will be done?
• Time: the schedule, i.e. how long the work will take and when it must finish. Question: how long will it take?
• Cost: the budget, based on resources, labour, equipment and materials. Question: how much will it cost?

Relationship between them

• If the customer adds features (scope increases), either the deadline moves out (time) or more people are needed (cost).
• If the deadline is shortened, either scope must be cut or extra cost (crashing, overtime) is needed.
• If the budget is cut, scope or schedule must give way, and quality can suffer.

Example

A college wants a student information system. If, after planning, the college asks for an extra online fee payment module, the developer needs more programmers (cost) or more weeks (time).

Use for the project manager

• Decide which constraint is most flexible and which is fixed.
• Evaluate every change request against all three.
• Schwalbe adds that a good project manager also considers quality, risk and customer satisfaction, since meeting only the three constraints does not guarantee success.

Modern PMBOK also considers further constraints such as quality, risk and resources, but the triple constraint remains the basic model.