The Triple Constraint in Project Management

Abstract

When managing a project several aspects constraints the project. These constraints are interdependent and the three key constraints form what has traditionally been known as the The Triple Constraint or The Iron Triangle. Some claim time, scope and cost (TCS) to be the key constraints others claim it is time, cost and quality (TCQ) . It is either way the project managers job to balance between the constraints to obtain trade-offs within a reasonable limit during the project's life cycle to obtain a successful project. The model implies that the project's successful completion is affected by the trade-offs performed and it is the project managers job to know how the constraints interrelate. The model however does not determine success . The model has been widely used since the 1950s and the application and limitations have been discussed extensively . Atkinson is one of the critics who claims the model is insufficient for determine the project success, because success rely on several additional factors and not only the three constraints . Others are more positive, but in general it seem that many would like to modify the model to improve the interpretation . Predominantly there seems to be a disagreement about whether scope or cost is the third constraint in the triangle. The aim of this paper is to explain the concept of the triplet constrain and it's application and limitations in project management and to reflect on the different perspectives.

Introducing the Triple Constraint

When managing a project there are several types of constraints e.g Cost, scope, schedule, resources and quality . In fact the PMBOK guide has divided the chapters about project management into exactly these constraints and how to manage them in a project i.e project scope management, project schedule management and so forth . These constraints are often interdependent of each other which is why it is important to balance the constraints against each other. According to ISO 12500 this means that a change in one constraint might affect the others . In project management the three key constraints are often considered to be either time, cost and scope or time, cost and quality. These tree constraints either (TCS or TCQ) form what is known as The Triple Constraint or The Iron Triangle (Figure 1 and 2).

Figure 1: The Triple Constraint model as presented by Van Wyngaard et al. (2012)

The concept of the triplet constraint has various interpretations depending on time period and conviction and has been widely used since the 1950s. The Iron Triangle discussed by Atkinson back in 1999 had quality, time and cost as the three constraints . Even though the many different interpretations, Van Wyngaard et al. (2012) claims the general consensus seem to be that time, cost and project scope form the tree key variables in the Triple Constraint (Figure 1) . The discussion is however still ongoing and some of the newest research by Pollack et al. (2018) claims the triple constraint consists of time, cost, quality (Figure 2) .

Figure 2: The Triple Constraint model as presented by Atkinson (1999) and Pollack et al. (2018)

Whether taking basis in the Triplet Constraint model with time, cost and scope or time, cost and quality the time characterises the duration of the project and the scheduling and the cost addresses the budget and resources (Figure 1 and Figure 2). The scope denote the tasks required to fulfill the project's goals and this could also be the quality constrain which denote the quality criteria set for the project. If The triple constraint model formed by these three constraints can be used to evaluate projects. The model states that the success of a project is impacted by those three constraints. During the project's life cycle it is the project managers job to balance between the time, cost and scope/quality to obtain trade-offs within a reasonable limit to obtain a successful project. This model implies that the project quality or successfully completion is affected by the trade-offs performed and it is the project managers job to know how the three constraints interrelate. It is however not possible to use the model to actually determine success, which is why the model is often used wrongly . Because of the mutual dependency between time, cost and scope/quality, a change in one of the constraints will affect the others making this model a useful tool in order to explain the effect or consequence of changes to project stakeholders.

The Three Constraints

As mentioned earlier are scope and quality very much debated. Some understand quality as a constraint others understand it as a success criteria for the finalized project. In the following is both scope and quality addressed as a constraint. The Triple Constraint is related to the definition of a project, but it is not as closely related as it was back when Atkinson (1999) wrote his article in 1999 . The international standard on project management define a project as: "A unique set of processes consisting of coordinated and controlled activities with start and end dates, performed to achieve project objectives" . This relates to The Triplet Constraint in the way that the activities need a start and end date meaning time is a core element in projects. At the same time the activities performed have to be controlled and coordinated in order to meet the project objectives, and this is basically the scope of the project and/or the quality criteria. To perform the activities resources are needed and hereby the cost.

Time

The definite beginning and end date for projects is related to the temporary endeavor. As described earlier the time constraint includes the processes required to schedule the project activities and in relation to this supervise and monitor the progress to keep track of the schedule. Often does the projects have a specific start and then the end date is estimated by the project manager. It can however also be the opposite case where the end date is determined and then the job is to determine the date where the project has to begin. A project is often divided into many sub tasks and activities. These activities have to be defined and sequenced and a duration has to be estimated in order to estimate the total project duration. All this can be considered as project schedule management . According to PMBOK Guide (2017) project schedule management includes the processes required to manage the timely completion of the project .

Cost

Normally projects cost money and the cost is related to the processes required to develop the budget and to monitor progress to control costs. In order to complete the project within an approved budget project cost management is needed. Project cost management includes the processes involving estimation of costs so the budget can be determined. When the budget is determined the cost has to be controlled so the project can be completed within budget.

Scope

The scope is related to the work and deliverables necessary to complete the project successfully and the processes required to identify and define these. Project scope management is very important no matter which type of project. If the scope is not controlled and resources are used on tasks that are not a part of the scope it can be difficult to deliver a finished project that is on time and under budged. As stated by the PMBOK Guide (2017) scope management is primarily concerned with defining and controlling what is, and what is not included in the project. Another important dimension in project scope management is how to be able to deal with and manage changes. During the project new demands can arrive naturally as the project evolve and stakeholders might have new and additional requests which changes the scope. The duration of each new task is critical to the successful completion of the final project . How to cope with stakeholders and to only accommodate those changes that are necessary to achieve the project goals and deliverables are important as changing the scope can have great impact on the cost and schedule because of the interdependence in the Triplet Constraint. For example if we have a project where the goal is to built a two storey family house the scope is then determined by the goal. The idea is that if we know what to achieve, it it possible to find a way on how to do it . In other words, the scope is defined by the goal. When the scope is determined the schedule and budget can be determine as well and the work can begin. If the client then asks for additional features e.g a balcony or an extra storey this will not surprisingly require additional resources. Extra work load for the team due to increase of the project scope will result in time and cost increase according to the Triple Constraint (Figure 1). If the budget was fixed the only way to decrease the cost is to decrease the scope, meaning no additional features in this case . It could also be the time that was fixed meaning additional resources in terms of people would have to work on the project resulting in higher cost in order to meet the scope. Because the three constrains are interdependent a change in one of them will result in changes in the others.

Quality

Both the ISO 12500 standard (2013) and the PMBOK guide (2017) mentions quality as one of the competing project constraints . The PMBOK guide (2017) describes quality management as the processes for incorporating the organization's quality policy and controlling the product quality requirements in order to meet the stakeholders expectations. But is quality A part of the Triplet Constraint? Some of the newest research by Pollack et al. (2018) have reviewed network diagrams and concluded that time, cost and quality were highly interconnected while scope was not even present in any of these networks. Implied that if the Triplet Constraint should be based on the most interconnected constraints it would consist of time, cost and quality . At the same time the paper argues that the relevance of the constraints whether if it is time, cost, quality, or any other criteria are entirely contingent on the context.

Application

The project manager will often during a project face continuously conflicting demands from stakeholders and competing priorities. During the project's life cycle it is the project managers job to balance between these constraints to obtain a successful project. As mentioned earlier the Triple Constraint model often require trade-offs performed by the project manager if additional requirements to either time, cost and scope/quality arise. It is therefore the project managers job to know how the constraints interrelate. The application of the model is to understand the mechanism of the interrelated constraints in order to obtain a successful project but it is not possible to determine success by using the model.

The Dynamics of the Triple Constraint

The dynamics of the Triplet constraint indicates that a change in one of the constraints will result in changes in the others because of the interdependence between them. In many projects it is common that one or more of the constraints are fixed. For example if the project has a fixed time constraint the scope/quality of the end product will depend on the cost and available resources. The same dynamics apply if instead the project scope is fixed. The cost will then largely depend upon the schedule and duration . According to Van Wyngaard et al.(2012) the inherent dynamics of the Triplet Constraint can be described by three key relationships (Figure 3) . In the figure is scope represented as the third constraint, but it could easily be replaced with quality and the mechanism would be the same, as the figure shows that any of the variables can be obtained at the expense of one or both of the remaining two variables.