Iron Triangle

Often used term in project management is the “iron triangle”, AKA the “project management triangle”, AKA the “triple constraint”. It is used to visually describe the relationship of three constraints on the project: cost, schedule, and scope. More recently quality is also added to the picture to show the effects of the three constraints of the overall quality of the project, as well as the risk that the project is facing. Most common representation looks like this:

Cost, Scope, Schedule + Quality & Risk

Conventional wisdom dictates that you can only “pick two”. For example, if you want to improve the schedule (make it shorter, faster) and improve scope (add more features, more output) you would have to sacrifice your cost (it’s going to be more expensive). This is a very popular principle and explanation of what is believed to be the main three project constraints. This is used to explain that all three constraints cannot improve at the same time and that at least one has to compensate for the other(s) to improve. If a PM gets a request that violates the iron trianagle principles, for example, when a sponsor requests an increase in output, without allowing change in cost or schedule, this principle could be the best way to explain why such a thing is likely not possible.

The iron triangle principle has been around for a while. Originally Dr Martin Barnes from UK has described the iron triangle in 1969 in his course material where he described the correlation of time, cost, and output (consisting of proper scope and quality). The relationship between the three constraints remain important however the visual representation is often misleading as the real relationship is more complex.

For the triangle to really work as a geometrical explanation the constraint values need to move in the same direction… As less cost and smaller schedule is good, less output is bad. This is further complicated by separating output into scope and quality. In that case, less scope could be a great thing if the level of quality can be maintained. This complexity was also recognized by PMI and the current PMBOK Guide does not include the iron triangle. However, what is in its place is even more complicated.

The reason why the iron triangle persists is that management, and in the PM case stakeholders and sponsors, often focus on specifically these three constraints. Their main concerns are:

  • What will get done?
  • How much will it cost?
  • When will it be done?

This is why the iron triangle will stay relevant for quite some time. The main goal of the PM as a change agent is to make sure that all the stakeholders understand that changes to any of the constraints will affect the other two (and them some). How exactly are the constraints affected depends on a lot more things than could fit on a single picture… and any suggestions are welcome! As long as this is understood the iron triangle will remain a powerful icon of project management.

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