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Is the Classic Triple Constraint in Project Management Still Relevant?

Triple constraint in project management

This is a guest post by Michelle Symonds.

Michelle Symonds

Michelle Symonds

Projects always have certain constraints – there may be an urgency to complete a project because of external market factors, for example; or there may have to be tightly controlled costs because of a fixed or limited budget. The project will also be expected to deliver on certain expectations including quality levels.

The most commonly used planning constraint method in project management was historically the “triple constraint” – time, cost and scope – sometimes known as the project management triangle or the “iron triangle” because it’s often shown as a triangle with the constraints at each vertex.

You get a glimpse of Martin Barnes, inventor of the Iron Triangle, in this video on the history of project management.

Alternatively, you may see it represented as 3 inter-locking circles with Quality at the centre.

Triple Constraints with Quality Diagram

But even including quality as one of the triple constraints in project management omits other constraints that have proved to be equally important. Factors such as Risks, Resources (human and otherwise), Expectations or even R&D or Technology Issues for ground-breaking tech projects.

Since managing project constraints is such a major part of effective project management many practitioners have expanded the constraints model to a diamond shape to include Quality as a key constraint. It has been increasingly recognised that even this does not go far enough and a 6-pointed (or more) star is proving to be a more realistic constraint model for today’s projects.

Why are Project Constraints Important?

Whatever shaped constraint model you choose to use and whatever constraints are deemed most important for any particular project what is always true is that if one constraint is adjusted it will undeniably have an effect on the others.

So if you have taken the simplistic approach that there are only three constraints – Time, Cost and Scope – and you increase the scope of the project then it is fairly obvious that this will impact the amount of time taken to complete the project and/or the cost of the project.

Equally, once you introduce Risks, Resources and Quality to the mix then changing just the scope again impacts quality, potentially introduces more risk and can also impact on resources.

It is balancing all of the constraints that is important to delivering a successful project – it’s not rocket science but continues to be a challenge on many projects.

Michelle Symonds quote

Read next: 5 Steps for Identifying Project Constraints and Dependencies

The Problem With the Traditional Triple Constraint in Project Management

It may still be possible to manage a project and deliver it successfully using the three simple constraints of time, cost and scope. Possible maybe on a simple project of a type in which the project manager and project team have previous experience, in which the costs are well understood and the scope is clear-cut. However, once you introduce an element of the unknown or a new team or project manager even simple projects can face difficulties with this model.

Introduce some complexity and then risks, resources, quality and expectations all become major influences over the success of the project and the triple constraint model can prove to be lacking.

As with so many other elements of project management, the models and methodologies provide a sound base from which to develop a more tailored approach for a particular project and its circumstances.

Different Constraints for Different Projects

An old boss of mine used to say that if the client was 100% satisfied then we had spent too much money, time and effort on the project

Although most PM practitioners broadly agree on the important constraints in any given project, the details of the constraints will naturally vary from project to project as will their importance to the stakeholders.

Some customers or stakeholders might be prepared to be flexible with the budget if they ultimately get what they want, whereas other customers may be prepared to compromise on the deliverables in order to stay within budget.

There is rarely a project where some form of compromise is not acceptable and many businesses with long experience of projects will know to expect compromises along the way. An old boss of mine used to say that if the client was 100% satisfied then we had spent too much money, time and effort on the project (but I won’t tell you who that was…).

For a project manager, then, it is important to understand the finer details of each constraint on each project and to manage accordingly. Customer expectation is perhaps one of the most difficult constraints to get right not just because expectations vary from customer to customer but because they might also vary between individual stakeholders – and can be hard to measure and clearly define.

Read next: 10 Quick Tips for Being More Customer-Centric on Your Projects

The Impact of Changing Constraints

Managing constraints on any project is about balancing each of them to deliver a successful outcome. Any changes will impact other areas of the project and this should be made clear to all stakeholders at the outset – the big question is really how and by how much will change impact the end result.

Changes cannot be overcome by throwing people and money at a project, even if you have the resources and the money – beware of thinking they can. Extra cash and resources can simply put pressure on other areas such as dependencies, not to mention the additional management time required.

The classic project management triangle with the three core constraints of time, cost and scope is still relevant but perhaps limited in its approach. It is certainly still a useful tool to focus priorities when making high-level decisions about how to best balance these constraints for a successful outcome.

About the author: Michelle Symonds is the Search Marketing Consultant for Parallel Project Training and writes about a range of project management and general business topics. She is the founder of Ditto Digital and also Editor of the PM news site Project Accelerator.

Photo credit: Photo by Samuel Zeller on Unsplash

Triple constraint in project management

About Elizabeth Harrin

Elizabeth HarrinElizabeth Harrin FAPM is a professional project manager and award-winning blogger behind A Girl's Guide To Project Management. She's passionate about demystifying project management and making tools and techniques work in the real world. She's also the author of several books including the PMI bestseller, Collaboration Tools for Project Managers.
Elizabeth lives in the UK with her family. She uses her organisation and project management skills at home, and also to help other bloggers at Totally Organised Blogging.


    I thank you for your brief explanation. But I have some questions.
    1. What are the challenges a PM faces in dealing with the triple constraints of a project?
    2. What trade-offs he/she has to make for the success of the project?
    3. I thank you again!

    • The challenges and trade offs are specific to the project and the sponsor/leadership of the project. I don’t think it’s possible to generalise about trade offs. All I could say is that you can use conflict management techniques and facilitation to deal with any problems that arise. You may find that you face no issues!

  2. Interesting points you are raising Elizabeth. A key point I took from your article, and comments above is around Customer Satisfaction as a constraint: could you give a hypothetical example on this?

    • OK, think of customer satisfaction as ‘value’. You can deliver a project on time, on budget, to the specified scope criteria, but still end up with unhappy customers (stakeholders). While it’s hard to articulate how you set boundaries for the constraint and control them, you can continually monitor stakeholder satisfaction during the project to make sure that you are meeting expectations and delivering something of value. It’s the subject of my book, Customer-Centric Project Management!

  3. I fundamentally disagree with the way this article describes the time, cost and scope triangle as constraints. For me the triangle describes the balance between the different success criteria for the project. It is used to negotiate between the stakeholders what are the objectives for the project. I need this thing by this time. These are agreed at a high level in the Business Case and then refined in the PMP. Some projects prioritise time whilst other prioritise quality. What Agile project do is fix the time element (and cost element) and then flex the scope with in the available resources. This makes it easier to agree on the objectives for a highly flexible project.

    Constraints are a different concept. They constrain the delivery of a project. The best example I came across was in building a school. You can not have heavy demolition machinery running during the exam season. Or you can not get access to a specialist resource. Or you have to use a particular programming language because it is company policy. Or you cant work on site whiles birds are nesting. These are constraints. As Christy says no amount of money is going to make it possible for you to disrupt people exams.

    So to summarise I could decide in discussion with the sponsor and project board that we could re-baseline the success criteria (delay the launce date, reduce the budget or de-scope the project) but it is not in the gift of the sponsor to change one of the constraints, such as chopping down trees when the birds are nesting or obtaining more of a unique resource.

    I hope this helps and I would be interested in getting you views too.

    • Thanks for your comments, Paul. That’s a great way of defining constraints. I wonder if we’ve ended up as an industry with vocab problem where ‘constraint’ is used informally (and not how you describe it) as shorthand for something to do with the context in which we want the project to succeed. The phrase ‘triple constraint’ could be the source of that, and it’s just something that’s passed into the language of PM in an unconsidered way. Just my thoughts.

  4. I think the one thing we all know about project management is that no two projects are the same. So even though we might use standard methodologies and follow best practices there is always room for some professional judgement by the project manager that takes into account the specific circumstances of each project. Indeed, I would hope that no PM simply blindly follows an approach without at least considering whether it is appropriate for a particular project.
    As Elizabeth says, customer satisfaction should also be in the mix of constraints – I’ve seen many a project where the defined constraints of time/cost/scope and quality are met on paper but the final deliverable just hasn’t been quite what the customer wanted – usually a failure to manage expectations…

  5. While I agree with the author on the fundamental message of this article – constraints for each project pose unique problems to that project & must be managed accordingly – I disagree that this means the triple or iron constraint (with quality in the center) is not still the best model at the essence of all of the issues mentioned. For example, expectations can be boiled down into an issue with one of the existing constraints…”I thought it would be cheaper” (Cost), I thought it would have done X, Y and Z” (Scope), I thought I would have had it already” (Time). Have a problem with resources that money can’t fix…say you can’t find the right expertise? That’s likely a problem with time, right? You don’t have the time to bring someone up to speed and the project requires a resource with the right knowledge *now* to stay on track. And risks? I rarely meet a risk that can’t be mitigated with money, and money not being in abundant supply means it is usually a cost constraint at the core.

    I feel adding lines to the triangle to make it a diamond, star, or decagon doesn’t help illustrate the challenges – keeping it simple for the stakeholders & decision-makers is much more likely to allow them to understand the issue & choices on the table. As PMs, it is our job to boil all of the issues down to their core to help stakeholders understand their choices. To this point, my favorite quote from this article is:

    “For a project manager, then, it is important to understand the finer details of each constraint on each project and to manage accordingly. “

    • Hi Christy. Those are considered comments, thank you for taking the time to write them. I see where you are coming from – and where Michelle is coming from. I agree that part of our role is to boil it down to the bare essentials for the sponsor and execs to help them understand. However, there is one I’m keen to see reflected in the constraint model that the time-cost-scope doesn’t adequately show (imho) which is customer satisfaction. If you go for a triangle with quality at the centre you could pop customer satisfaction in with quality. But personally I think it should be totally explicit.
      Having said that, the bottom line is using your professional judgement to understand and manage accordingly, just like you both said.


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