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)