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What is Triple Constraint?

One of the oldest and most useful concepts in project management is triple constraint. Sometimes referred to as iron triangle or project triangle, triple constraint captures the reality that in order for a project to achieve its required objectives and meet quality expectations, it must successfully operate within three boundaries: scope, cost and time.

Triple Constraint Model is About Trade-Offs

The triple constraint model acknowledges that a major piece of the project management puzzle is about making trade-offs. For example, a project that is falling behind schedule could be accelerated by adding more staff. However, this would increase costs — which may or may not be possible or acceptable. Or consider a troubled project that is not achieving all of its expected objectives. Extending the schedule could solve this problem, but it will result in missed deadlines (and possibly higher costs as well).

Frankly — and not to alarm any aspiring project managers — it is extremely rare to be involved in a project that does not involve making several trade-offs with respect to scope, cost and/or time. Or to put this more bluntly: it is not a matter of if a project will crash into at least one (but likely two and probably all three) boundaries in the triple constraint model, but when. The art and science of project management is deeply rooted in the ability to navigate these trade-offs.

Now, let us take a closer look at the three elements that make-up the triple constraints of project management: scope, cost and time:

Project Triple Constraint Element: Scope

Scope refers to everything that must be done for a project to achieve its required business outcomes, and meet stakeholder expectations (we explore the importance of meeting expectations further in this article). Scope also includes identifying what should not be included in a project.

Achieving consensus with respect to scope — what is included, and what is not included — can be difficult; especially on larger and more complex projects. However, failing to clarify and establish scope prior to the launch of a project will inevitably lead to significant problems down the road. In some cases, these problems will be so destabilizing that the project has to be re-invented or shut down.

In its Project Management Body of Knowledge ((PMBOK® Guide – [2021]), the Project Management Institute (PMI) lists six processes that are part of scope management:

  • Plan Scope Management: processes related to creating a scope management plan that includes how scope will be defined, verified and controlled.
  • Collect Requirements: processes related to establishing, documenting and managing stakeholder requirements and needs, in order to meet project objectives.
  • Define Scope: processes related to building a detailed description of the project and product (i.e. what the project aims to deliver).
  • Create WBS: processes related to creating a hierarchical breakdown of all the work products that must be performed for the project to achieve its deliverables and accomplish its objectives.
  • Validate Scope: processes related to formally accepting the completed project deliverables.
  • Control Scope: processes related to monitoring project status and scope, and managing changes as necessary to the scope baseline (i.e. the original scope prior to project launch).

Project Triple Constraint Element: Cost

In a perfect world (at least for project managers!) projects would have unlimited budgets to pay for all necessary fixed and variable costs, and take care of unforeseen expenses that invariably occur (indeed, for many experienced project managers their two favorite words are “contingency buffer”). Of course, budgets are always limited, which is why cost management is vital for project success — and in the big picture, project survival.

The PMI’s Project Management Body of Knowledge lists four processes that are part of cost management:

  • Plan Cost Management: processes related to establishing how project costs will be estimated, budgeted, managed, monitored and controlled.
  • Estimate Costs: processes related to identifying an informed estimation of the financial resources required to complete all project work.
  • Determine Budget: processes related to combining all of a project’s individual activities in order to create a cost baseline (i.e. the original cost estimates and budget prior to project launch).
  • Control Costs: processes related to monitoring project status, updating costs as necessary, and managing changes to the cost baseline. </>

Project Triple Constraint Element: Time

One of the defining characteristics of all projects — and a fundamental aspect that separates projects from programs — is that they have definitive start and end points. Granted, sometimes these end points are many months or even several years in the future (e.g. London’s crossrail, California’s high-speed train, China’s South-North water transfer project). However, whether the goal is to launch a marketing campaign or move millions of people a year, there is an underlying schedule — or rather, a volume of schedules — that governs when tasks occur and in what order.

The PMI’s Project Management Body of Knowledge lists six processes that are part of schedule management:

  • Plan Schedule Management: processes related to establishing project schedule policies, procedures, and documentation.
  • Define Activities: processes related to identifying and documenting what actions are necessary to produce project deliverables.
  • Sequence Activities: processes related to identifying and documenting relationships among project activities (i.e. when they need to occur, and how work on one activity impacts other activities).
  • Estimate Activity Durations: processes related to estimating the amount of work periods required to complete individual activities using the estimated resources.
  • Develop Schedule: processes related to analyzing the sequence, duration, resource requirements, and constraints of project activities, in order to create the project schedule.
  • Control Schedule: processes related to monitoring the schedule, updating the schedule as necessary, and managing changes to the schedule baseline (i.e. the original schedule estimates prior to project launch).

Triple Constraints of Project Management and Agile

At first glance (and perhaps second and third as well!), it may seem that triple constraints of project management and Agile methodology are mutually exclusive. After all, Agile is inherently iterative and dynamic, and constraints are not just unwelcome, but in some cases they can be fatal.

It is true that Agile projects must be fluid and capable of pivoting in response to customer expectations. However, it is also true that Agile projects — just like all other projects — have a limited amount of budget and time (whether these are sufficient or insufficient is beside the point: the fact is that they are both finite). And while the scope evolves through each sprint, it must nevertheless adhere to the project charter and user story.

As such, while the triple constraints in Agile project management are relatively less severe and more fluid than those in traditional (i.e. waterfall) project management, they are still part of the overall picture, and must be identified, managed and optimized accordingly.

Triple Constraint in Project Management and Risk

Another important, yet sometimes misunderstood aspect of triple constraint deals with risk. Since the word “risk” is typically referred to in a negative manner, some people — especially those who are not project professionals — view risk as a constraint. This perception is understandable, but inaccurate.

Risk is not an inherent constraint. However, avoiding risk is a constraint. Keep in mind that avoiding risk is not the same thing as mitigating risk. The former is refusing to acknowledge that risk exists. The latter is about acknowledging risk, and then developing and implementing strategies and tactics to reduce the likelihood and severity of negative risk, and increase the possibility and benefit of positive risk.

Best Practices for Applying Triple Constraint Model

Below are some practical best practices for applying triple constraint model in project management:

  • While it is valuable to learn from previous successful projects, it is important to remember that what may work in one type of project or vertical may not fully translate into another. Triple constraint is a paradigm, not a prescription.
    Using exceptional portfolio and project management tools is mandatory vs. optional. Focus on a solution that is cloud-based, so that internal and external stakeholders can participate (to the degree and extent that their specific role should allow) and remain in the loop.
  • The definition of “project success” can — and often does — vary from stakeholder to stakeholder. Achieving consensus on this issue before a project launches is critical. One of the most practical and effective ways to do this is by developing a robust Project Charter that (among other things) clearly defines project success, and clarifies how it will be measured.

Project Management Diamond

Even when project success is clearly established and agreed upon by all relevant stakeholders, perceptions can change during the course of a project; especially one with multiple phases, or which has run into difficult challenges. For this reason, some project managers have adopted a modified version of triple constraint called the “project management diamond”. As illustrated in the graphic below, this model highlights that managing and achieving sponsor/stakeholder expectations is at the heart of project success, even more than quality.

The Final Word on Triple Constraint

Triple constraint is a practical and functional way to grasp that in project management, trade-offs with respect to scope, cost and time are a big part of the story — and ultimately play a core role in defining, developing and ultimately determining project success.


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