The Triple Constraint in Project Management
When managing a project several aspects constraint 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 determination of the project success, because success rely on several additional factors and not only the three constraints . Others are more positive, but in general it seems that many would like to modify the model to improve the interpretation . Predominantly there seems to be a disagreement about whether scope or quality 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 ConstraintWhen 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 on 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).. 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) .
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. 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 . The international standard on project management defines 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.
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 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 .
Normally projects cost money and the cost is related to the use of resources and the processes required to develop the budget and to monitor progress to control cost. 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.
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 budget. 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 build 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.
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. This 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.
The project manager will often during a project continuously face 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 requires 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 ConstraintThe 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.
Within the trade-off mechanism is the notion according to Van Wyngaard et al. (2012) in general, that project managers are constrained to chose two of the three elements and in order to gain the chosen two the third is sacrificed. This is also known as: "Good, fast, or cheap? Pick two" . This principle is used to describe Figure 3 in the following . The first relationship in Figure 3 shows that if it is desired to achieve the scope or increase it, it requires that time and/or cost increases. If it is imagined that the cost is fixed, there are two outcomes for the project, either the project to be "cheap" because of the fixed cost or "good" because of increment of scope but the project can never be fast because of the increasing time. The second relationship in Figure 3 explains the effect of reducing the duration of the project. In order to reduce the time and make the project "fast" , a reduction of the scope together with a possible larger cost is necessary. Again we have two outcomes if we imagine that the scope is fixed. A "fast" and "good" project can be obtained because of the fixed scope but the project will never be "cheap" because of the larger cost. The third relationship in Figure 3 explains how the effect of reducing the cost affect the project. If the cost is reduced it necessitates a reduction of the project scope and/or a longer duration of the project. A fixed time gives rise to a "cheap" project and a "cheap" project because of the decreased cost, but then the project will not be "good". The relationships can be used as a guideline when understanding how the trade off between the constraints work. It can also be used to explain how additional demands from stakeholders affect the project. It is however important to keep in mind that there may be other aspects constraining the project and that the relevance of the constraints might depend on the context.
The Triple Constraint model has been widely used since the 1950s and the application and limitations have been discussed extensively. Back in 1999 Atkinson criticized the model of being too locked and not being able to follow the ongoing development of the project management phenomenon . He believed the problem back then was that while project management evolved in many areas the success criteria remained unchanged because of the close relation to the project definition. He considered the possibility of why projects fail as a result of the success being measured by a limited set of criteria to match which he claimed to be "two best guesses and a phenomena". He suggested a new model "The Square Route" to understand success criteria which he believed to be a better and more balanced and realistic indication of success . The "Square Route" includes success criteria which benefit the organization and stakeholder community such as improved efficiency and effectiveness and satisfied users personal development and so forth. But is it in fact necessary to extend the Triplet Constraint to all these sub categories? It would be possible to regard all these additional success parameters as a part of the defined quality/scope constraint. Maybe it is just a matter of how specific you are about your quality and scope constraints as a project manager and how much you listen to and involve your stakeholders when constraining the project and measure success using the Triplet Constraint model. It might also depend on the type of project, it could be that some types of projects are better suited for the TripletConstraint model than others.
Today we have a definition of a project in the ISO 21500 which does not directly contain the the criteria of time, cost and quality and yet the model is still debated . In line with Atkinson it is often claimed that the model is way too simple as a measure of project success and that it is neglecting other important constraints and that the model is not aligned with the actual lived experience of projects . It is stated that the Triple Constraint model simplifies the challenges and difficulties that will occur and will need to be addressed during a project. Van Der Hoorn & Whitty (2015) claims it can be critical for inexperienced project managers to use the triple constraint model because the focus will be to manage time, cost and scope/quality and everything else will be neglected which they consider as simple thinking . When it becomes to simple it is easy to forget the complexity of a project and the higher purpose. Even though Van Wyngaard et al. (2012) in general seem positive about the triple constraint model a modification to the model is still proposed . The "TRIJECT" model is proposed to support/consolidate the Triplet Constraint model and connect it to the higher purpose of the project which is claimed to be missing in the traditional Triple Constraint model as also Van Der Hoorn & Whitty (2015) claims . The "TRIJECT" model forms a continuous cycle which represent the ongoing process that runs when a change is introduced to the system . But is the simplicity exactly what makes the triplet constraint useful? The broad categories of time, cost and scope/quality can basically contain everything you could imagine as a success criteria which you could argue is actually a very flexible model. It is just a matter of how specific you define you project. The risk is of course if you are an inexperienced project manager it can be hard to keep track of all these sub categories of success criteria which implies in the broad categories of time, cost and scope/quality. As mentioned earlier the Triple Constraint model is useful to understand how the trade-off mechanism works but it can not be used to actual determine success which you also have to keep in mind when using the model. The PMBOK guide describe the projects managers role as someone who has the ability to develop and manage scope, schedules and budgets so maybe it is not that important whether we use one model or the other as long as we remember the interdependence between the constraints .
There does not seem to be a clear agreement whether scope or quality should be the third constraint in the model. Which constraint to chose as the third could very well depend on the type of project and the complexity of the project The traditionally thought of a Triple constraint model is constantly challenged and accused for being inflexible, too simple and not useful to measure success as a project manager. Even though others are more positive many would like to modify the model. What is not negotiable is that the project manager will have to balance between different constraints during the project life cycle and the constraints will affect each other due to the interdependence of the constraints.
 Pretorius, J. H. C., Van Wyngaard, C. J. and Pretorius, L. Theory of the triple constraint– a conceptual review.IEEE International Conference on Industrial Engineering andEngineering Management, 13:1991–1997, 2012. This article reviews some of the different point of views concerning the Triplet Constraint which are used in the following article to provide different approaches on how to use these constraints and understanding the trade-off mechanism when managing a project. This article does also describes the dynamics of the Triplet Constraint concept and provides a consolidated model (TRIJECT-model)to improve the interpretation of the Triplet Constraint Model.
 Pollack, J., Helm, J., and Adler, D. What is the iron triangle, and how has it changed?International Journal of Managing Projects in Business, 11(2):527–547, 2018. This article provides some of the newest research about project constraints and is interesting for the following article in relation to the discussion about whether the third constraint is scope or quality.
 Westland, J. The triple Constraint in Project Management: Time, Scope Cost. https://www.projectmanager.com/blog/triple-constraint-project-management-time-scope-cost, 2018. [online] Accessed: 02-16-2019. This site briefly describes what has been considered as the traditional practice of the Triple Constraint and the interrelations between the three constraints. This has been used in this article to outline what the basic concept of the Triple Constraint is.
 Atkinson, R. Project Management: Cost, time and quality, two best guesses and a phenomenon, its time to accept other success criteria. International Journal of ProjectManagement, 17(6):337–342, 1999. As the title of this reference implies is this a critical article from 1999 about the Triplet Constraint. In this article Atkinson argues that theTriplet Constraint concept is outdated and not being able to follow the development of project management. This article is relevant because it provides an interesting point ofview upon how project success was defined formerly and it is interesting to see that the application and limitations of the Triplet Constraint are still discussed today.
 Danish Standards.DS/ISO 21500:2013 - Guidance on Project Management. DanishStandards, 2. edition, 2013. ISO stands for International Organization for Standardization and this particular standard provide guidance on project management. Citations from this standard primarily refers to section 2 and 3.11. Section 2 concerns terms and definitions used in project management e.g the definition of scope, cost and quality. Section 3.11 provides the standard’s definition of project constraints.
 A Guide to Project Management Body of Knowledge. Project Management Institute, 6.edition, 2017. PMBOK developed by PMI (Project Management Institute) is a guide that describes the knowledge about project management, both in terms of what has been proven traditional practice and new practices introduced because of the ongoing evolvement of the project management profession. The PMBOK guide provides the foundationfrom where the organization can build procedures, techniques, tools etc. This reference is used to get the project management perspective on the Triple Constraint and how it is suggested to manage those constraints.
 Marchewka, J. T.Information Technology Project Management: Providing MeasurableOrganizational Value. John Wiley Sons, Inc., 2003. This book is about providing measurable organizational value and how to measure success for projects. Only page 31-34 of the book is used as reference in this article. The cited chapter is about the context of project management and how trade-offs between different constraints have to be made when managing a project.
 van der Hoorn, B.,Whitty, S.J. Signs to dogma: A heideggerian view of how artefacts distort the project world.International Journal of Project Management, 33:1206–1219,2015. This article provide interesting perspectives on the Triplet Constraints and how to measure project success used in the following article to provide different point of views.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Van Wyngaard, C.J., Pretorius, J. H. C., Pretorius, L. Theory of the triple constraint – a conceptual review. IEEE International Conference on Industrial Engineering and Engineering Management, 13:1991–1997, (2012).
- ↑ 2.0 2.1 2.2 2.3 2.4 Pollack, J., Helm, J., and Adler, D. What is the iron triangle, and how has it changed? International Journal of Managing Projects in Business, 11(2):527–547, 2018, (2018).
- ↑ 3.0 3.1 3.2 Westland, J. The triple Constraint in Project Management: Time, Scope Cost. [Online] https://www.projectmanager.com/blog/triple-constraint-project-management-time-scope-cost, Accessed: 02-16-2019, (2018)
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Atkinson, R. Project Management: Cost, Time and Quality, Two Best Guesses and a Phenomenon, it's Time to Accept Other Success Criteria. International Journal of ProjectManagement, 17(6):337–342,(1999).
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Danish Standards DS/ISO 21500:2013 - Guidance on Project Management. Danish Standards, 2. edition, (2013)
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 A Guide to Project Management Body of Knowledge Project Management Institute, 6.edition,(2017)
- ↑ 7.0 7.1 Marchewka, J.T. Information Technology Project Management: Providing Measurable Organizational Value John Wiley & Sons, Inc., (2003)p.31-34
- ↑ 8.0 8.1 8.2 Van Der Hoorn, B., Whitty, S.J. Signs to dogma: A Heideggerian view of how artefacts distort the project world International Journal of Project Management, 33:1206–1219, (2015).