One of the first things project management students learn is that a project’s cost, time, and scope are interdependent. These three project constraints form the iron triangle. If one constraint changes, the other two change as well. For example, adding extra features to a product costs more and takes more time. Although the iron triangle was a measure of project success for decades, it is no longer so. Yet the iron triangle is not obsolete.
At the beginning of the project management era and for many decades to come, the iron triangle measured project success. Successful projects balanced time, cost, and scope—they were completed on time and within budget. However, even some completed projects that exceed time or budget can be perceived as successful by project stakeholders. Also, a project completed on time and within budget can be a failure if it leads to team burnout, for example. It is clear that more than just balancing the iron triangle contributes to a project’s success. For example, the PMBOK (v. 4) says a project needs to balance other constraints besides time, cost, and scope, namely quality, resources, schedule, and risks.
However, the iron triangle can still offer guidelines for setting the success criteria of a project. The project manager can continue to use it to understand stakeholders’ most important constraints for that project and help stakeholders agree on what constraint they can afford to “slide.” With that information, the project manager can plan the project better. In almost any project, the iron triangle tends to go out of balance because scope, cost, and time and their interrelationships are complex, thus difficult to predict and more difficult to control.
Scope can change during the lifetime of a project. This is intrinsically true for an agile project, whose scope is only broadly defined at the beginning of the project, but can also be true for traditional (waterfall) projects, due to changes such as market trend variation, domain evolution, and new legislation. For example, in IT, the cost and time of a project are two sliding constraints for about 50 percent of projects, according to statistics.
Some project management practitioners propose the use of a flexible, or agile, cost-time-scope triangle because an iron triangle is not realistic since the three constraints are difficult to control. A flexible triangle means that the project manager needs to convince stakeholders to be flexible with project cost, time, and scope—of course, while staying within certain boundaries—starting from the premise that the three constraints will slide and the iron triangle will become unbalanced during the project’s lifetime.
Many project managers who question the iron triangle’s relevance to project success are probably right. Balancing the iron triangle does not guarantee the success of any project, traditional or agile. However, the iron triangle remains a useful tool for helping stakeholders choose the most relevant project constraints and understand how a change in any constraint can affect all the others as well as the project.
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