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What is the Agile iron triangle?

To understand the relationship between Agile and the iron triangle, we will explore three separate definitions:

  • The traditional iron triangle
  • The Agile iron triangle
  • The Agile triangle

The traditional iron triangle

This iron triangle is the original model used to represent three chief constraints in project management:

  • Time
  • Cost
  • Scope

It is also known as the project management triangle or the triple constraint.

The concept of the iron triangle was introduced by Dr. Martin Barnes in 1969. All three factors are dependent on one another, which aligns with the traditional project management frameworks of Waterfall. In this methodology, the project scope is fixed, and the time and cost will work around this.

The Agile iron triangle

The term ‘Agile iron triangle’ could be considered an oxymoron, as the nature of iron itself is inflexible. However, it can be used to describe the adaptation of the iron triangle for Agile teams. How does this work? By flipping the triangle upside down

Due to the involvement of the customer throughout the development process, the scope of an Agile project can change. Time and cost are more likely to be fixed as deliverables need to reach a client by the end of an Agile iteration and remain within the proposed budget.

The Agile triangle

Jim Highsmith, one of the signatories of the Agile Manifesto, tweaked the factors of the iron triangle to make it more suited to Agile practitioners. He proposed three different angles:

  • Value
  • Quality
  • Constraints

The three original constraints still exist here, but they are combined into one section. Instead, more emphasis is placed on the Agile pillars of value and quality. The project deliverables must provide noticeable value to the customer — to ensure this, Agile teams create user stories written from the customer’s perspective. Quality is assured through regular testing and refining, in line with the iterative approach of Agile software development.

Highsmith’s model can also be referred to as the Agile project management triangle or software development triangle.

So, which one of the three triangles should a project manager opt for? As mentioned, the traditional iron triangle works for projects with a non-movable scope. An example could be a construction project, such as building a garden shed. Meanwhile, both the inverted triangle and Highsmith’s triangle could be used for a variety of Agile projects where the customer’s required deliverables can change. One example could be a software development team designing a new company website for a client.

Should you use the traditional iron triangle in Agile?

Though the original iron triangle concept has been widely embraced, many believe that its traditional roots are not suited to Agile, and it must be altered to stay relevant.

The Project Management Institute (PMI) argues that the traditional iron triangle is limited, and there are other factors that a project manager must contend with outside the areas of scope, cost, and time. This coincides with the PMBOK Guide’s 2008 addition of three new constraints:

  • Quality 
  • Benefits
  • Risk 

The PMI adds that the ‘hard’ factors of the original iron triangle should be integrated with the ‘soft’ factors of the soft pyramid — “a metaphor for concurrent constraints related to the ‘internal satisfaction’ of the individuals working on the project” — to create a more holistic approach to Agile project management.