Scheduling: Critical path, PERT and Gantt

Developed by Frank Almirudis, currently under construction

Title to be changed to: Choosing critical path, PERT or Gantt as your project scheduling method

Abstract

Under the aspect of Complexity in project management, Scheduling is the development and control of a plan which details when and how the products or services defined in a project scope will be delivered.

The resulting plan is known as a project schedule, which can also be used as a tool for management of stakeholders' expectations, means of communication and/or evaluating performance. Project data like planned dates, activities, resources, constraints, dependencies and activity sequence and duration are all inputs to the project schedule, and should be constantly monitored and updated by the project management team. To properly create and use the project schedule with the aforementioned inputs, the team must select the scheduling method that best suits the needs of their project.

Three scheduling methods are covered in this article:

• Critical path method (CPM)
• Program evaluation and review technique (PERT)
• Gantt

The focus of this article is to help the reader:

• Know the definition, theory and purpose behind each scheduling method.
• Understand under what circumstances each method is best applied.
• Decide which scheduling method fits their case best.
• Understand the limitations of their selected method.
• Reference the reader to different sources on how to apply their selected method.

Scheduling methods

Figure 1: PERT/CPM diagram example.

Figure 2: Gantt chart example.

Critical Path Method (CPM) is a technique created to identify the key activities that are critical for the completion of a project, bearing in mind that if any of these were to be delayed, then the whole project delivery would be delayed. The end game of this method is to find and optimal balance between the length of the project and it's associated costs, as well as identifying the costs of the activities are minor according to the optimal time. In order to create a critical path diagram, one must at least consider all the activities that are requied to deliver the project, their duration and their dependencies. As these components are similar to the ones used in PERT, the two methods are often used together and their combination is called PERT/CPM.

Program evaluation and review technique (PERT) is a scheduling method aimed to create a project's timeline in a graphical manner. This timeline helps the user monitor and control the project's activities while ensuring that the interdependencies between said activities are kept in check and complied to in order to deliver the project. While activities are the central component of PERT, they are not scheduled, this is because the interdependencies dictate when an activity is able to be completed, as all other activities preceding it must be fulfilled before the next activity may be done. PERT diagrams are integrated by 4 key elements: Activities, preset date, calendar and contrains.

• Activities are sets of errands that are to be done to complete the project, these should have a completion time and a description.
• Preset date are set to the activities that may critically affect the delivery, and these dates are aimed to be respected.
• Calendar of working and non working days to accurately respresent the time when this project is to be completed.
• Constrains to set the interdependencies of the activities, these contratins can be: finish-to-start, finish-to-finish, start-to-start or finish-to-finish.

The timeline using PERT is usually represented by using two methods: Activity On Arrow (AOA) in which each activity is represented by arrows, while nodes are starting and finishing events of the activities. The other method is Activity On Node (AON) where activities are repesented as nodes while the arrows are the interdependencies of the activities. Note that an activity may not be started until all preceding connected activites are fulfilled.

Gantt chart is a visual representation that showcases tasks, ther dependecies and time constrains that allow project managers to identify challenges, control them and come up with different solutions to said challenges, using vertical and horizontal axis to represent a simple timeline for a project. The horizontal axis indicates the time available in a project, while the vertical axis represents the different tasks that need to be performed to do a project in the expected time delivery. Dependencies within each activity can be set as well in this method, however, because of how the visual representation is done in this particular method, adding too many dependencies may generate visual clutter, which can cause confusion to the viewer.

Indicators to consider when choosing a scheduling method

To properly assess if one should use PERT/CPM or Gantt for scheduling their project, the project management team must consider a number of indicators regarding the project. These indicators help the team understand where their project stands in a different number of fronts, and by understanding the project's priorities they can decide in a more objective way which method to use.

Figure 3: Gantt chart used in high complexity projects.

Complexity

The first and most important indicator to take into account is complexity of the project. The number of activities and the level of dependency between said activities will be in most cases the deciding factor in choosing between PERT/CPM and a Gantt chart. While a Gantt chart is a tool which almost anyone can use to generate a project schedule and it's easy to follow, when the activities and their dependencies start to pile up the Gantt chart gets too complex and it's hard to read and follow, losing the most fundamental advantage of the Gantt diagram over PERT/CPM, Figure 3 showcases an example of a Gantt chart used in a situation where the activities and dependencies were too many. An example on complexity can be seen in the Engineering, Procurement and Construction (EPC) process of a thermal power plant. Because of the level of detail that the EPC contractor has to look into in order to deliver a thermal power plant, they are likely to use PERT/CPM to monitor the various dependencies within the project, where as suppliers of specific equipment for the power plant (e.g. a pump manufacturer) will be more inclined to use a Gantt chart, due to the fewer steps in getting a single piece of equipment manufactured. The bottom line is: If the project management team considers that their project is simple enough in terms of activities and dependencies then a Gantt chart should be considered. If the complexity is too high for a Gantt chart, then the reader should consider using PERT/CPM, taking into account the below indicators as well to ensure that they are getting what they want out of their project schedule.

Resources available to generate the project schedule

Another front that has to be taken into account is the different resources available to generate and update the diagrams. PERT/CPM, while being a more robust scheduling method, takes significantly more resources in order to generate a diagram. It is often done using specialized software and has to be done by a qualified person. Because projects with great complexity generally create more revenue, the money for software and qualified personnel is often not an issure, however, it is important to consider another resource which is time. Time management is also an issue, as it will take longer to develop a PERT/CPM diagram than it will take to generate a Gantt chart. The reader must carefully consider the time available to generate the schedule plan. If the resources are not available, it's adviced to reconsider using Gantt charts.

Need for flexibility

How much uncertainity exists in the project and how likely is it that activities within the project will change? These are important questions to consider when choosing a scheduling method. This is due to the flexibility of said methods. While PERT/CPM is costly and time consuming to generate, it is also easy to come back and adjust when the project. This is due to the fact that the method not only accounts for the activities that are the most critical to deliver the project, but also the surrounding activities, meaning that if the context of the project was to change, simply adding more nodes of activities or events and updating the corresponding dependencies and dates would be enough to have the project schedule back in track, this makes PERT/CPM an attractive choice when there is risk, since Gantt charts basically have to be generated from scratch each time there is a significant update. Flexibility is one of the strongest points for PERT/CPM.

Stakeholders who will need to read the schedule

During the execution of the project, multiple times there will be needs from stakeholders asking for a progress report of the project, it may come in the form of a client wanting to know when they will receive their deliverables, an executive from your own company wanting to better understand the point that the project is at, or simply someone from the team who wants to know when his activity has to be deliver. The scheduling method of your choice may or may not be able to help you with these kind of situations. Gantt charts take the lead in this category, as anybody can interpret them. PERT/CPM will not be legible to people outside of your project management team, even if an outsider is profecient with PERT/CPM, most times it takes significant investment to understand how the person who generated the plan structured it, meaning that only people who are constantly engaged with the plan (project management team) will understand it. It is important to take this into account, as choosing to go with PERT/CPM will inevitably create need for additional work in progress reporting in the future.

Dependencies clarity

How clear is the method at showcasing the dependencies of each of your activities, this section feeds back into the complexity aspect of the project. Gantt chart is good at portraying the dependencies between the activities as long as there is not an overwhelming amount of them. PERT/CPM on the other hand won't have a problem presenting these interconnections of activities to the project management team. This is another strong point for PERT/CPM.

Timeframe clarity

Timeframe is often an overlooked aspect of the scheduling methods, even though the single most important thing about a schedule is time keeping. PERT/CPM does state the due dates of each of the activities, but it fails to create a visual guideline of how time is running throughout the project. The project management team has to be consistent in assessing the dates and when a particular task is coming closer to critical state, where as Gantt charts don't have this problem, as the horizontal axis on them are a representation of time, making them particularly good at giving immediate visual feedback to the viewer about what deadlines are getting ever closer. What a lot of project management teams end up doing if they are using PERT/CPM diagrams is to create Gantt chart for small sections within the PERT/CPM diagram. This gives them the visual timeline and therefore become much more aware of which activities deadlines are starting to become more important.

Resource allocation

Finally, although both methods do a poor job when it comes to showcasing resource allocation, Gantt chart does include owners of each activity, meaning that the activities have someone in charge of making sure that they are fulfilled and it can be easily observed when looking at the diagram. PERT/CPM generally does not incldue any form of resource allocation.

To conclude, when choosing a scheduling method, it is important to note the different indicators listed above. Complexity in the form of quantity of activities and the level of their interdenpendency is by far the most important indicator, but the others mentioned have to be taken into account in order to properly assess if a project management team should consider either PERT/CPM or Gantt as their scheduling method. Generally when the teams have a highly complex project in their hands and opt to use PERT/CPM, they also fall back to have Gantt charts for smaller sections of their project due to the advantages that they possess over PERT/CPM diagrams and how easy they are to read and generate.

Limitations

Complexity is a clear limitation for Gantt chart, but it is also a limitation to PERT/CPM when the amount of activities and interdependencies are of great amounts (in the thousands). Resource allocation is also a great limitation of the scheduling methods covered in this article as they basically don't cover this region in their methodology, even though it is an area that has direct impact when it comes to the overall planning of a project. In terms of individual methods, PERT/CPM sees its limitations in the lack of a clear timeline, the resources needed to generate the initial model and the legibility for an outsider of the project management team, therefore affecting it's ability to be used as a communication tool. Meanwhile, the biggest limitation of the Gantt diagramm, other than problems with complexity and resource allocation, is its need for constant rework and inflexibility, since any significant change calls for a new Gantt chart to be produced.