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Understanding Task Sequence Dependency Attributes in Project Scheduling

by | reviewed 07/03/2021

By Mark Romanelli, MBA, CAPM

In the Project Schedule Management knowledge area, one of the steps in the planning process involves sequencing project activities. This means arranging them in order and deciding what should happen, when it should happen, and considering it all in relation to the other scheduled activities. The order in which activities happen is called the activity sequence.

Section of the PMBOK Guide, 6th Edition clarifies some of the ways in which activity sequencing comes to be. Sequencing attributes are explained according to two separate dimensions; Mandatory vs. Discretionary and Internal vs. External. Based on which characteristics are attributed to the sequence, the sequence can be classified as one of four different types. This concept is often overlooked by new project managers, but it shouldn't be. Knowing and understanding the different types of sequence dependencies provides a lot of benefits. Foremost, it helps a Project Manager scheduling a project to better understand the schedule, which in turn leads to improvements in the overall quality of the schedule and its creation process. Further down the line, understanding these kinds of dependency attributes allows a Project Manager to identify opportunities to compress the schedule, when doing so becomes necessary as it often does for a multitude of different reasons.

The following points in this article explain the key concepts important in activity sequencing, explains their application in easily understandable terms, and provides some tips for gaining a deeper understanding of the topic.

The sequencing of most project activities is determined by certain attributes of the dependency between and among the activities. Dependencies are the relationships among tasks, activities, or other schedule items that determine the order in which activities need to be performed. In determining activity sequencing, it needs to be considered whether or not activities need to happen in a particular order. Some activities need to happen in a certain order. Some don't and can be scheduled at any time. Others still can be performed in parallel, occurring at the same time.

The first dimension to examine is whether a dependency sequence is mandatory or discretionary. The second dimension asks if that dependency is internal or external.

Mandatory Dependency – This is a dependency ordering that has to take place and can be thought of as a, "must." Mandatory dependencies are also known as hard-logic and are fixed limitations, often determined by physics, legal requirements, or other non-flexible elements.

Discretionary Dependency – Discretionary dependency orders aren't physically, legally, or otherwise fixed, but still considered to be a good idea to keep. "Should," is the best word to keep in mind when thinking about this type of dependency. They are considered discretionary because they are at the discretion of the project team who decides to implement them.

Internal Dependency – Internal dependencies are dependencies resulting from forces internal to the organization conducting the project. They can be mandatory or discretionary and include processes, procedures, practices, or rules imposed by or created from within the organization.

External Dependencies – These sequence dependencies are determined as such by forces outside of the organization undertaking the project. Laws, safety standards, principles of physics, and other dependency creators outside of the organization's control are examples of external dependencies. As with internal dependencies, external dependencies can be mandatory or discretionary.

A great way to understand the concept is to apply it to a familiar topic. The process of getting and eating a pizza can be considered, by definition, a project. As such, the steps and activities involved in the process should all be familiar to us. We can use the sequence of activities among some of these steps to illustrate examples the four sequence dimensions.

  • Mandatory Dependency - You can't slice a pizza until it is cooked. If you try to do it, the pieces will bind back together in the oven. Therefore, this dependency sequence of cook then cut is a 'must.'
  • Discretionary Dependency – You can't eat the pizza until after it cools down. Of course it is physically possible to eat the pizza directly after removing it from the oven. However, since the pizza will be very hot, this is not a good idea. Since it is something you decide you 'should' do, it is discretionary.
  • Internal Dependency – As a matter of personal policy, you only have pizza for lunch or for dinner. There is no law or medical dietary restriction for this and some people do, in fact, eat pizza for breakfast. Still, you have come to this policy on your own and can change it if you so choose.
  • External Dependencies – If you order a pizza to be delivered, you can't eat the pizza until it is first delivered by the restaurant. This sequence is dependent on the outside force, in this case the restaurant, which is external to your force of control.
These attribute dimensions can be combined to create four distinct styles of dependency sequencing attribute types:

The Four Dependencies in Project Scheduling

  • Mandatory-Internal
  • Mandatory-External
  • Discretionary-Internal
  • Discretionary-External

In order to better understand and be able to apply the concept to your next scheduled project, I recommend a little practice exercise. Think of a field that you know well, perhaps even the field that you usually manage projects in. Next, consider an example of each of the four sequencing dependency attributes; Mandatory, Discretionary, Internal, and External. Then, take it a step further and think of a sequence dependency example for all four of the combination types: Mandatory-Internal, Mandatory-External, Discretionary-Internal, Discretionary-External. The more familiar you are with the types, the better you will be able to apply them in scheduling your next project.

About the Author: Mark Romanelli is a full-time lecturer in the Sports, Culture, and Events Management program at the University of Applied Science Kufstein Tirol (FH Kufstien Tirol) in Kufstein, Austria. His curriculum includes courses in Project Management and Strategic Project Development. He is a member of the Project Management Institute and a Certified Associate in Project Management.