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Project management triangle: overview of the triple constraints

Also called the iron triangle or project triangle, the triple constraint refers to these three elements of a project:

  1. Cost
  2. Scope or quality
  3. Time

The theory is that you have to compromise on at least one of these things, which creates a trilemma. You might know this from hearing some variation of the cliché “fast, cheap or good: pick two”. Of course, a bad project will compromise on all constraints, overrunning on time and budget, and then delivering a bad final product.

Project management triangle example

Let’s take an example of a client that wants an ecommerce site. Here’s how the iron triangle impacts this project.

Compromising on time

“I want this website to support purchases and I want it done cheap.”

“We’ll need to make the website secure and compliant with the law. If you’re not going to pay for more help, people will have to double up on work, and that will take time.”

Compromising on scope/quality

“I want this website done cheap and quick.”

“If you’re not paying much, we’ll have to either rely on junior developers or really cut back on what the website can do, possibly just using a template site with limited ecommerce features.”

Compromising on cost

“I want this website to support purchases and I want it done quick.”

“We’ll have to either employ more staff or push our other work out of the way. You’ll have to pay for that, or it won’t be worth it for us.”

Why does the project management triangle sometimes use scope and quality interchangeably?

Scope and quality aren’t always the same thing. A simple project with a limited scope can still have extensive quality control. Some alternatives to the triple constraint do distinguish between scope and quality. Others put quality at the centre of a time/cost/scope triangle, like so:

Three triangles labelled cost, scope and time, with a triangle in the middle labelled quality

The reason scope and quality are often used synonymously is that customers think in terms of deliverables. Deliverables are defined as ‘qualities/features that make the product fit for purpose’. To customers, the product is higher quality if it has more working features. So if you think of scope as ‘what the project team can achieve’, it makes sense for quality to also fall under that definition.

Alternatives to the project management triangle

Project management diamond

The project management diamond is essentially the triangle, but it lists scope and quality as two different factors.

A diamond with the points labelled cost, quality, scope and time

At the centre, people will usually put either ‘expectations’ or ‘customer satisfaction’. Either way, this model prioritises the customer, rather than the product itself.

Project management star

The PMBOK® Guide uses a popular alternative model to the triple constraint. It lists six constraints, made from two overlapping triangles in a star shape.

A 6-pointed star with the points labelled schedule, risk, scope, quality, budget and resources

As you can see, the project management star also distinguishes between scope and quality. Here, it says scope is constrained by the budget and schedule, while quality is assured by managing risks and resources.

How to manage the triple constraints

Of the three constraints, it is much more important to manage scope. This is why:

Scope lets the team to know what they’re doing. Even if the deadline is short or the budget is lacking, the project manager can still keep everyone informed, which is necessary to progressing the project.

Managing scope creep. Clients can change their minds at the last minute, especially since they’re more likely to understand the consequences of cutting the budget or moving a deadline. They may not realise the harm they do when they move the goalposts.

Withstanding sudden changes in a project. Bringing in more people or throwing more money at the project can actually slow it down. So along with compromising the budget, this can hugely impact the time and scope.

How to determine and apply the triple constraint

This depends entirely on the type of project. The three project constraints need to be discussed in detail with the client before the project starts. They need to know what it will cost in terms of time and scope, as well as budget. You’ll also want to establish that there won’t be any sudden or drastic changes during the project.

Remember that a bad project compromises on all three constraints, and that throwing money and extra time at a project often makes things worse. So stakeholder communication is key. Everyone needs to be fully aware of how the project complies, as the three constraints apply to and affect everyone.