The Iron Triangle: Is it Time to Improve the Triple Constraint? [update]

The Iron Triangle, or Triple Constraint, is one of the best-known ideas in Project Management. It competes with the Gantt Chart. To non-practitioners, it’s not as widely understood, but it is more fundamental.

We’ve been using it for years and it has many names – of which Iron Triangle is my least favorite by far! I prefer Triple Constraint. But, call it what you will, is it still fit for purpose? Or do we need to improve it, overhaul it, or even abandon it altogether?

UPDATE: This 2024 update includes the Seven Aspects of Project Performance from the latest (7th edition) of the PRINCE2 best practice guide.

Let’s start with an introduction. This is one of the very first videos I made for our OnlinePMCourses YouTube Channel:

Contents

In this article, I’ll address five key points:

• The History of the Triple Constraint
• Understanding How the Triple Constraint Works
• Alternative Iron Triangles: What Three Constraints?
• Alternatives to The Triple Constraint: Different Aspects of a Project
• My Take on the Iron Triangle: Let’s Continue to Love the Triple Constraint

Oops… Did I just give away the punch line? Happily, this isn’t a joke or a novel. Let’s get started.

From the Barnes Triangle to the Iron Triangle: The History of the Triple Constraint

I need to credit Adrian Dooley’s excellent short article in the Summer 2022 edition of Project (the quarterly magazine of the Association for Project Management) for my understanding of some of this history. Also, Adrian, if you’re reading: we are 100% aligned on Triple Constraint being the best name!

‘The common law of business balance prohibits paying a little and getting a lot – it can’t be done’

John Ruskin, 1819-1900

Or, as my father used to say, ‘you get what you pay for’! So, this idea is not new. Indeed, there seems to be no clear origin for the widely used axiom:

‘Good, fast, or cheap: you can pick two’

The Barnes Triangle

But, as I learned from Adrian, the modern form of a triangle dates to 1969 and Dr Martin Barnes (a founder, Chair, and President of the Association for Project Management). He illustrated the three competing tensions of time, cost, and quality as a triangle.

Since then, we have used many names for it:

• Barnes Triangle
• Time Cost Quality Triangle
• Project or Project Management Triangle
• Triangle of Balance (which helps explain an important idea)
• Triple Constraint (which helps explain another important idea)
• Iron Triangle

The Iron Triangle

The Iron Triangle has become the most widely-used term. It isn’t clear to me where it first appeared, nor when it morphed from representing Time, Cost, and Quality to Time, Cost, and Scope.

Barnes did make a shift to referring to ‘Performance’ rather than Quality, which arguably includes scope. But I think focusing on Scope is a retrograde step, which I’ll discuss below.

But I do appreciate the ‘iron triangle’ metaphor. Triangles are rigid in that they cannot readily be deformed. And nothing conveys that rigidity better than ‘iron’. We think of girders and bridge structures.

Iron Triangle is both an apt metaphor AND a misleading one.

It is apt because it reminds us that we cannot escape the triangle.

But it is misleading. Because the truth is that we can (and do) stretch the triangle. We can have more quality if we are prepared to pay more. And we can save money if we are prepared to cut corners. And we can get an earlier delivery if we either invest in more resources or cut quality (or scope).

Understanding How the Triple Constraint Works

All of this is about how the Triple Constraint works as a powerful thinking tool. The three corners of the triangle constrain one another.

Triangle of Balance

But let’s start with another helpful name for it: the Triangle of Balance. I like this name because it reminds us that we need to bring the corners into balance. With the right budget, we can deliver to the required quality (or scope) in the agreed time. If we want to change one of these, we must change at least one other, to bring the triangle back into balance.

Triple Constraint

Another way of saying this is that the three corners constrain one another. If we want to change one corner, we can. But only if we ease off on one or both of the others. The three corners are a triple constraint.

Alternative Iron Triangles: What Three Constraints?

You’ll notice that I have been flip-flopping between Time-Cost-Quality and Time-Cost-Scope articulations. Both are, I guess, equally valid. And it’s my sense that:

• Time, Cost, and Quality is more widely used in the UK
• Time, Cost, and Scope is more widely used in the US

I have no idea about other places, so please do feed my knowledge in the comments below! My guess is that preferences correlate to some extent with the cultural proximity to these two countries.

In the next section, I’ll show two ways that we can resolve this by incorporating both. And we should consider them as being different, because they are:

• Quality answers the question, ‘How good do you want it?’
• Scope answers the question, ‘How much of it do you want?’

Combining Scope and Quality

However, there are at least three ways that we can combine scope and quality into one coherent concept, to make a triangle of either:

• Time, Cost, and Performance
  Which Barnes used
• Time, Cost, and Specification
  Where we specify both scope and quality
• ‘On time, on budget, on target’
  Which I used in the sub-title of my book ‘How to Manage a Great Project’

Popular Alternative Triangles

You’ll have your own favorites, but I like:

• Schedule, Resources, Specification
  Here, schedule is just time, and ‘resources’ is a wider concept than cost, since money is just one resource, alongside labor (people), assets, equipment, and materials
• Risk, Benefits, Investment
  This is widely used in Program Management as the major levers of control, where time, budget, and scope morph and expand over a longer timeframe
• Value, Quality, Constraints
  This is the ‘Agile Triangle’ where the term ‘constraints’ represents time, cost, and scope (sneaky, hey!)

I have a video that explains the Agile Triangle…

Alternatives to The Triple Constraint: Different Aspects of a Project

As soon as you start to like some of the additional possible ‘corners’ of the triangle, you start to want new shapes.

The Project Diamond… or Tetrahedron

The first step is to include quality and scope on an equal footing with one another, to make a diamond shape:

However, this fails to capture the sense of constraints and balance. A diamond shape is easily deformed. For that reason, in my training, I represent it as a tetrahedron (triangular-based pyramid. This has the same rigidity as a triangle and therefore better represents the need to balance the four corners.

Let’s Go Large! The PMBOK Guide, PRINCE2, and PRINCE2 Agile

The PMBOK Guide (4^th Edition) and PRINCE2 each introduced a set of 6 things to consider:

PMBOK Guide

The PMBOK Guide refers to six competing constraints:

1. Schedule
2. Budget
3. Resources
4. Risk
5. Quality
6. Scope

PRINCE2

At the same time, the new (autumn 2023) edition of PRINCE2, PRINCE2 7, refers to seven variables, or seven aspects of project performance:

1. Benefits
2. Costs
3. Time
4. Quality
5. Scope
6. Sustainability
7. Risk

Of these, ‘sustainability‘ is the new addition for PRINCE2 7 – and the reason for updating this article! PRINCE2 defines sustainability in terms far wider than ‘just’ the environment – important as that is. It uses the United Nations 17 Sustainable Development Goals as its framework, which extends well into the realm of social value.

Comparison

Apart from superficial differences in terminology, there are two big differences that make the PRINCE2 articulation superior.

1. PRINCE2 includes benefits.
   Despite the commendable emphasis on Value that the PMBOK Guide 7th edition has, the authors fail to see this as a constraint. My guess is they see it as a target (which it certainly is). However, there’s a balance to be had. If you want more value, you have to optimize costs and benefits against one another.
2. PRINCE2 includes sustainability.
   This is not the first time that PRINCE2 authors have led the conversation. This is such a valuable inclusion. It’s one of the things that made PRINCE2 7: Managing Successful Project my PM Book of 2023.

And it is also worth noting that the PMBOK Guide chooses Resources. I see this inclusion as redundant, since the list also includes Budget. Money is the master resource. It pays for all other resources (people, assets, equipment, materials, or software.

More on PRINCE2 7

If you want to know more about PRINCE2 7, check out:

• My article, PRINCE2 7th Edition: The Top 10 Changes You Need to Know
• My YouTube LiveStream replay, PRINCE2 7: What, Why, How, and Should You?
• The Global Best Practice Handbook, PRINCE2 7: Managing Successful Project.

From Triangle to Heptagon

And, from Iron Triangle to Hafnium Heptagon.

Hafnium is a silvery white metal that is extremely resistant to heat and corrosion. Yet it is highly ductile and so can be drawn out into very fine components.

PRINCE2 Agile

PRINCE2 has a whole section about the different types of project tolerances, that refers to its six aspects of project management.

But PRINCE2 Agile takes this a whole lot further. It suggests that it is in the nature of agile projects that we fix time and cost. There is no tolerance to flex them, because they become committed when we establish the length and resourcing for an iteration or sprint. Indeed, one of the five ‘targets’ of PRINCE2 Agile is:

‘Be on time, and hit deadlines’

(A little tautological, if you ask me)

You may well choose to fix or flex benefits and risk. We start from a minimum acceptable benefit and a maximum risk tolerance. But, within those constraints is room for adaptation.

In agile, PRINCE2 Agile asserts, it is quality and scope that we constantly flex, by prioritizing according to the needs of our customers or users. Another of the five ‘targets’ of PRINCE2 Agile is:

‘Accept that the customer does not need everything’

However, there is another target that appears contradictory:

‘Protect the level of quality’

When you have chosen the priority to give to quality and specified the standard that the customer needs, quality becomes fixed again.

I wonder, in light of the recent PRINCE2 7th edition, whether we’ll see an update on PRINCE2 Agile soon. Will the authors consider Sustainability a ‘flex’ or ‘fix and flex’ variable?

The FLEKS Model

In his Fleks Model, Hélio Costa describes 6 Value Variables that match the PMBOK Guide constraints closely:

1. Time
2. Cost
3. Resources
4. Risks
5. Quality
6. Scope

However, Fleks represents the six variables as orbiting around value.

We use them to determine how to optimize value creation, and so are all subservient to this critical variable. If we need more value, we ‘fleks’ one or more of these six things. In Fleks, ‘everything is variable’!

We have a detailed article by Hélio on this website and a full interview with him too. And, of course, you can download all his free (open-source) materials from fleksmodel.com.

Please note that, at the time of writing this update, Hélio Costa is reviewing Fleks and we can expect to see a new version shortly after this article is published.

My Take on the Iron Triangle: Let’s Continue to Love the Triple Constraint

With so many enhancements over the years, it’s fair to ask whether the simple Triple Constraint of Time, Cost, and either Quality, Scope, or Specification is still useful. Or does it, perhaps, oversimplify what’s important in the delivery of a project? This Is especially the case when we are working in environments where:

• All projects are hybrid projects with anything from a few to a lot of adaptive processes, methods, and tools
• The line between a ‘big project’ and a ‘program’ is becoming increasingly blurred.

Is the Triple Constraint Fit for an Agile Age?

I think the answer depends on what you think the Iron Triangle is for. Is it for:

• Establishing the success criteria for a project?
• Understanding Project priorities?
• Balancing competing constraints?
• Optimizing value from the choices we have available?
• Articulating what is important in project performance?
• Determining tolerances for variation and change?

Without a doubt, for each of these uses, the longer lists add something to our understanding and flexibility.

Why I Love The Triple Constraint

Yet, I fundamentally believe that the Triple Constraint is not down and out yet. It retains its value to us.

Because, for me, its real value is as a thinking model and a teaching tool.

Explaining the need for balance and the mutual mesh of constraints provides deep insights. And exploring the range of variables and when each is important helps us develop a greater sensitivity to the details of projects in general, and our current projects in particular.

The Triple Constraint in The Agile Triangle

And, if you need any further persuading, take one last look at the triangle in the Age of Agile. The Agile Triangle may look different, but what’s that I see in the bottom right-hand corner?

Do you see it?

Agile evangelists may sound like they want to reject all things predictive, but there it is. At nearly 60 years old, embedded in the Agile Triangle, is the Iron Triangle – a variant of Martin Barnes’ original insight.

For more about the Agile Triangle, check out my video, What is the Agile Triangle – Value, Quality, and Constraints | Video.

What are your Thoughts on the Iron Triangle?

So, what do you think? Please share your thoughts on the different versions of the Iron Triangle and its bigger – maybe better – cousins, in the comments below. And I will respond to every contribution.