What are the Triple Constraints?
For successful delivery, project management practitioners balance three constraints: scope, cost and schedule. These constraints are often represented as a triangle and consequently the triple constraints are also known as the Iron Triangle. It has many representations, one of which is shown below:
The triple constraints were originally conceived as a framework to enable project managers to evaluate and balance the competing demands of cost, time and quality within their projects.
Interpreting the Triple Constraints
The triple constraints provide an interesting approach that could be used to help keep a project on track. In simple terms, a project management practitioner needs to ensure that projects are delivered within budget and on time, meet the agreed scope and meet the defined quality requirements.
A simple example can be useful to explain how these constraints impact decision-making. Suppose you are releasing a product on schedule, a competitor has released their product and now you are under pressure to deliver before the planned date. In this case, the schedule needs to be pulled forward, impacting the other constraints. Most likely, the cost will go up because you will need more resources (although sometimes adding more resources can actually lead to delays).
Alternatively, you could reduce the scope of work, such as not deliver non-critical features and still deliver within budget and cost. This simplistic example shows how project stakeholders need to be aware of the triple constraints and the impact. The project manager needs to juggle these constraints as per stakeholder expectations to deliver successfully.
Limitations of the Triple Constraints
Though useful, it is rather simplistic for the complex environment in which projects are delivered. To understand the limitations of the triple constraints, let’s take another example. Suppose you have delivered a project on time, within budget, met the agreed scope and met the defined quality requirements. Is that an indication of a successful project? Going by the Iron Triangle, it is!
However, in real projects there are other factors or constraints, such as the results after deployment. The priority of constraints is also a variable that needs to be factored. This variable is sometimes dependent on the industry. For example, in NASA and military development, performance is a critical variable. Cost and schedule are not as important.
It is because of such limitations that project practitioners question the validity of the triple constraints as the de-facto framework to manage and evaluate projects. That’s probably the reason for the PMBOK version 4 to state that managing a project involves managing constraints, such as Scope, Quality, Schedule, Budget, Resources and Risk. It is also the reason for the triple constraints to evolve into the Project Management Diamond and the Project Management Star.
- Andrew Marshall and William Meckling, 1959. Predictability of the Costs, Time, and Success of Development, P-1852.
- PMI, 2009. A Guide to the Project Management Body of Knowledge: PMBOK Guide, 4th ed. Project Management Institute.
- Image Source: Wikimedia Commons - The Project Management Star per PMBOK
- Wei Lee, 2010. Manager’s Challenges and Managing Constraints
- Jonas Balshoej Ebbesen and Alexander J Hope, 2013. Re-imagining the Iron Triangle: Embedding Sustainability into Project Constraints.