Project Management: What is the ‘Triple Constraint’ model?
The project management theory suggests that projects, whatever their purpose, goal or type, are typically constrained by three key elements: scope, time and cost. The Triple Constraint model is one name used to describe these elements, another popular name being the Project Triangle.
The elements of the Triple Constraint model are variable by name. For example, cost is sometimes being referred to as budget, and time being referred to as schedule. However, their meaning remains the same:
- Scope: The goals and deliverables required to fulfil a project.
- Time (Schedule): The schedule for the project to be fulfilled.
- Cost (Budget): The financial constraints of the project.
The concept of the Triple Constraint model is that if one of the constraints is affected, at least one of the other constraints needs to be adjusted to accommodate the change. In other words, these constraints are related in a defined way and are predictable.
For example: if the scope of the project is expanded, the project’s schedule and cost will need to be increased to accommodate for that. If the schedule is shortened, the project’s cost will need to be increased etc.
In addition to the three elements explored above, quality element is often presented in the Triple Constraint model. Quality is considered a central theme within the Triple Constraint model, because any changes to the main three elements are believed to affect the quality of the project’s deliverables.
For example: if the schedule of the project is shortened, the project’s quality will suffer.
Newer versions of the modeli
Increasing number of digital projects, had an impact on the Triple Constraint model and the newer version was introduced with quality element becoming a fourth key constraint, as shown on the picture on the right. Project Diamond name is used to describe this version of the Triple Constraint model.
Customer satisfaction is considered a central theme within this model. This is because digital projects are considered to be successful if the customer expectations have been met or exceeded.
The concept of this newer version remained the same: if one of the constraints is affected, at least one of the other constraints needs to be adjusted to accommodate the change.
It was argued that there are more constraints to the projects, which has led to the introduction of another version of the Triple Constraint model – the Star model (two overlaid triangles as shown on the picture on the right).
According to this model, the key project constraints are:
• Time (Schedule),
• Cost (Budget),
• Resources, and
The concept of the Star model: if one of the constraints is affected, it is highly likely that at least one of the other constraints will have to be adjusted to accommodate the change.
The Triple Constraint model is rarely right. The theory claims that scope, time and cost constraints are related in a defined way and are predictable, however, it does not demonstrate a true relationship between constraints in practice. This is highlighted in the educational paper by Angelo Baratta (2006) which calls the model a “triple illusion”.ii
Angelo poses the questions “Why is it that the majority of projects that are over budget are also late?” and “Why is it that projects that are both late and over budget also have a tendency to under deliver (scope)? Again, this is not consistent with the Triple Constraint.”ii
The answer to his questions is simple – the triple constraint model, including the newer versions, are not perfect. The Triple Constraint model should not be perceived as a rule, but rather as a possibility.
The latest version (Star model) accommodates better for this issue and suggests that it outlines the key six constraints, rather than all of them. This is in addition to the statement that if one out of the key six constraints is affected, it is highly likely that at least one of the other constraints will be impacted.
This version provides a better representation of a project constraints and the theory recognises the changing nature of constraints throughout a project lifecycle. Therefore, it provides a good constraint management foundation for projects, suggests the importance of ongoing management of these key six constraints, and notes the likelihood of a constraint’s change having an impact on other constraints.
Putting constraints management into practice
The Triple Constraint model (Project Triangle) is certainly a classic, but it is by no means perfect. The Star model provides a better representation of a project constraints and highlights the importance of ongoing management of key constraints throughout a project lifecycle:
• Scope management,
• Time (Schedule) management,
• Cost (Budget) management,
• Quality management,
• Resources management, and
• Risk management.
It is a Project Manager’s responsibility to manage and balance competing project constraints throughout the projects. There are a number of tools and techniques available for Project Managers to help them put effective management of each constraint into practice.
For example: GANTT Chart and Critical Path Method are highly effective for schedule management.
It is vital for current and aspiring Project managers to have an in-depth practical understanding of effective constraints management and relevant tools and techniques. Content of the project management certifications enables candidates to gain the relevant knowledge and skills in this area. Therefore, it is highly recommended for current and aspiring Project managers to pursue a relevant project management qualification.
Lastly, please do not hesitate to contact SkillPoint to discuss certifications which would enable you to fast-track your Project Management career.
i Smartsheet. The Triple Constraint: The Project Management Triangle of Scope, Time, and Cost.
ii Baratta, A. (2006). The triple constraint: a triple illusion. Paper presented at PMI® Global Congress 2006—North America, Seattle, WA. Newtown Square, PA: Project Management Institute.