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02 January 2011

The Triple Constraint and Beyond

David A. Zimmer
David A. Zimmer, PMP
Chief Project Professor
American Eagle Group
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Practical Definition
The factors which help the project manager deliver a successful project. One constraint drives the priorities while another supports the delivery of those priorities.


Defining the Triple Constraint

The three factors by which we can define our project. They are Time, Money and Scope. Some say Schedule, Budget, and Scope. Or any combination.

Description

The Triple Constraint, sometimes referred to as the Iron Triangle, represented the three major determining factors of a project.

Time or Schedule is the length of time for a project. Sometimes, the length is determined by a hard, pre-defined or drop-dead date. If the project finishes after the date, the result is useless.

For example, if you are planning a party and the cake and gifts arrive a day later, they did not meet the time required. The guests are gone and the usefulness of the cake is diminished.


Sometimes, the time duration is determined by the sequence of events and the accumulated duration for each event in their order of execution. For example, if it takes 20 minutes to wash a car, 10 minutes to dry it and 1 hour to wax it, then the total duration for the project is 1 hour 30 minutes assuming no breaks or wasted time between tasks.

Money or Budget defines the cost of the project.  There may be an upper limit as to how much can be spent in order to achieve the desired or defined results.

For example, if you only have $300,000 to spend on a house and can’t possibly find a penny more in the budget and the house you want costs $350,000, you have some decisions to make concerning the features to eliminate or pick a different floor plan.


Scope defines the deliverables that must be produced to meet the project requirements. Anything less constitutes an unfinished or incomplete project. Anything additional defines things not necessary for the project.


For example, if the plans call for a five car garage and the builder only builds three, the house would be considered unfinished and unacceptable. If the builder adds an additional bay, then the timeline and budget would be impacted.


These constraints relate to each other. For example, with more money, we can typically add more to the scope, e.g., a bigger house with more bathrooms. With more time, we can be concerned about the details so each item is completed to our exacting standards. With less scope defined, we need less money and time.


The Triangle

Usually, the Triple Constraint is drawn as an equal-lateral triangle. In order to keep the angles the same, as one leg of the triangle lengthens or shortens, the others must do the same. As the budget is cut, the scope must decrease along with the time. (I know, I hear you saying, “In your dreams, buddy. Get into the real world.” I know because I’ve been there more times than I like to remember, but just stick with me.)

While it is interesting to know about the Triple Constraint, how does it really help me in managing real projects?


Driving, Weak and in the Middle

The key concept to know is the relationship between the constraints in respect to the key stakeholders' expectations or desires. We label the constraints as
  • Driving Constraint
  • Weak Constraint
  • Middle Constraint
The Driving Constraint drives the project. If the budget is the most inflexible, i.e., it can’t be increased, we must meet that budget to be considered successful (see Successful Project Definition). The Driving Constraint is the constraint our key stakeholders are the most unwavering on. We must meet the Driving Constraint.

Additionally, we must know the Weak Constraint. The Weak Constraint is the constraint with the most flexibility. It is the one we wiggle around on in order to keep the project on time and on budget. If the budget is the Driving Constraint and the Scope is the Weak Constraint, then we as the project manager might recommend a change in scope in order to hold to the budget. We might decide some features can wait until another time or they are not as important as preserving the budget.


Buying a car might be an example of the budget driving the purchasing project while the car features might be the most flexible during the negotiation process. We might be willing to forgo certain luxury features to keep the monthly payments within our means.
The Middle Constraint is just that. It is more flexible than the Driving Constraint and less flexible than the Weak Constraint. 

Who Decides Which is Driving and Which is Weak

Unfortunately for us project managers, we don’t pick the driving, middle or weak constraints. The key stakeholders do that for us. We must learn from them what is most important and which is the most flexible concerning the three: Time, Money and Scope.
If your stakeholders are anything like mine, and I suspect they are, simply asking them which one is the most important will only yield, “All Three!” But the ole project management adage states, “You can have it good, you can have it fast, and you can have it cheap. Pick two!” One drives the project and one is “flexible” – within reason, of course.

Usually, stakeholders are not forthcoming in offering hints suggesting which the driving or weak constraints are. Fortunately, sometimes the project provides some ideas. Sometimes a date becomes our deadline such as a budget freeze, drop-dead date, etc. Sometimes the budget is set by the size of the grant, the amount of the loan or other factors. But many times, nothing finite is apparent which leaves us interviewing the stakeholders trying to discover, based on their answers and reactions, the weighting of the constraints. The interview process is a critical business skill in itself and beyond the scope of this article (regardless of time or budget.)


The Dark Side of the Triple Constraint

There is a downside of the Triple Constraint. It would be nice if their priorities remained the same throughout the entire project. Unfortunately, this is not so. On top of that, many key stakeholders change their minds, project factors change (budget cuts, company priorities change, customers modify their orders, fickle markets adjust, etc.) and a myriad other reasons. An astute project manager must be sensitive to such “alignments” to shift project resources and focus accordingly.

To add insult to injury, we typically are not warned or informed of such shifts. We typically learn about the changes after the fact. Therefore, we must be proactive and through continual contact with the key stakeholders and probing them with questions, ascertain the constraint priorities. If the priorities change, the project manager must adjust accordingly.


Using the Knowledge of the Triple Constraint

So, how do we use the Triple constraint knowledge to our advantage and manage our project according to the stakeholders’ priorities? 

Every project ebbs and flows, just like a river. Sometimes, progress is advancing without hindrance and other times, it hits bumpy waters like rapids. Other times, it slows to a crawl. Leveraging the Triple Constraint, we shift resources to tasks to get the progress back in line. If time is the driving constraint, we put resources on the tasks that could delay the project time line. If we can burn through a few more dollars, adding additional resources might be a better option. If eliminating some features of a project or delaying their implementation until a later release will keep the project time in line, we make the recommendations for the changes.


If budget is the driving constraint and time is the weak constraint, we hire cheaper labor with less experience, and therefore, take longer to perform the tasks at hand.

The Driving Constraint dictates the focus of the project manager while the Weak Constraint provides the leverage to align the project with that focus.

But Wait, There’s More…

In the Fourth Edition of the Project Management Institute’s Project Management Body of Knowledge (PMBOK®), the guide for industry standard project management practices, the authors expanded the Triple Constraint by stating there are more constraints impacting projects. On page 6, they list
  • Scope,
  • Quality (a level of quality must be met requiring more time and money),
  • Schedule (overall length of project or forced downtime such as plant closings, weather delays, etc),
  • Budget (limited funds, budget cuts or funds unavailability),
  • Resources (a resource may not be available when the schedule states causing a delay in the overall project), and
  • Risk (space program must launch at specific time to lower risk of weather patterns could delay launch, etc.)
Any one of those can drive a project’s focus. Regardless of which constraint above becomes the driving constraint, the project manager must strive to understand the weak constraint(s) to keep the driving constraint on course.

Conclusion

The Triple Constraint, or its new variant – Sextuple Constraint – is the project manager’s best friend. Understanding their relationships to the key stakeholders’ priorities help the project manager deliver successful projects. Knowing which constraint determines the project’s success, we can leverage other factors to meet the requirements.

Managing projects is never easy or foolproof. But sometimes, having just one tip in our pocket aids us in getting it home safely.


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