Gantt chart and Scheduling techniques

Abstract

According to the Businessdictionary scheduling is about "determining when an activity should start or end, depending on its (1) duration, (2) predecessor activity (or activities), (3) predecessor relationships, (4) resource availability, and (5) target completion date of the project."

In this article, a review of scheduling techniques Gantt chart and Critical path method (CPM) will be presented. The Gantt chart is one of the oldest scheduling tools and is a graphical way to explain and display the schedule-related information. The tool displays each activity and the duration, and clarify what should have been achieved at a certain point in the time. The limitations with the Gantt chart is that the tool does not show the critical activities, which the Critical path method (CPM) does. The CPM is an algorithm for scheduling a set of project activities in the project. In the CPM a Network diagram needs to be developed. The Network diagram is about identifying the component activities and their relation to each other.
This article will provide a step-by-step guide on how to create and use the mentioned scheduling techniques. Furthermore, this article will explain how the different scheduling techniques can supplement each other, and what the limitations in each scheduling tool are.

Project Time Management

Figure 1: Scheduling overview. Received from "Project management body of knowledge (pmbok® guide)"

Project Time Management provides the processes that are required in a project with a timely completion. The Project Time Management includes six processes which are described below :

1. Define Activities - Identify the activities and actions to be performed to produce the deliverables of the project.

2. Sequence Activities - Identify and document the connections between the activities in the project.

3. Estimate Activity Resources - Estimate the amounts and types of the material, people, equipment, or supplies needed to perform the activities in the project.

4. Estimate Activity Durations - Approximate the number of work periods required to perform individual activities with the expected resources.

5. Develop Schedule - Analyse the activity durations, sequences, resource requirements, and schedule restrictions to produce the project schedule.

6. Control Schedule - Control the status of the project to update project development and managing the changes in the project schedule.

Scheduling A plan can only show the practicability of accomplishing its objectives when the actions and activities in the plan are put together in a schedule that illustrates when each action or activity will be carried out. Today there is many different methods and approaches to scheduling. It can either be by computer-based planning tools or done manually. The scheduling method dictates the procedures and rules for the scheduling process in the project. Tools like Gantt Chart and Critical Path Method (CPM) are tools often used in the planning process of a project. By combining the scheduling tool or method and using the outputs from the processes in the Project Time Management; define activities, sequence activities, estimate activity resources, and estimate activity, the project schedule can be produced. Figure 1 describes how the scheduling methodology, scheduling tool, and outputs are interacting with each other and in the end creates the project schedule.

The Gantt Chart

The Gantt Chart is the oldest form of a formal scheduling tool. The Gantt Chart was developed in the 1910s by Henry Gantt, and today used widely as a scheduling tool in projects. A Gantt chart keeps track of the progress of the multiple activities there often is in a project.

Definition "A graphic display of schedule-related information. In the typical bar chart, schedule activities or work breakdown structure components are listed down the left side of the chart, dates are shown across the top, and activity durations are shown as date-placed horizontal bars.".

Gantt charts are especially useful in one-time scheduling projects. For example projects like building construction or making films are both projects that are one-time projects. Overall, the advantages of the Gantt Chart can be realised in all types of projects and organisations.

How to create a Gantt Chart?

Developing a Gantt Chart takes some steps. In the scoping phase determines the level of details and the activities identifies in the project. There are many ways to create a Gantt Chart, and in the end, it depends on the project and the working style. In the table below some general steps to developing a Gantt Chart are described.

How to create a Gantt Chart. Received from "Smartsheet" [1]

Creating a Gantt Chart step by step

Step 1 - Creating the axes: Create a sheet with vertical and horizontal axes. The vertical axis represents the activities, and the horizontal axis represents the timescale. To make the Gantt Chart more clear and get and an idea of the total time that needs for each activity, adding the calendar time in timescale is a good idea, see figure 1. Step 2 - Creating the bars: Create bars representing each activity. The length of the bars should be proportional to the duration time. If there are multiple activities it is a good idea to create a summary or hammock activity, see figure 1. By using hammock activities the project can be summarised in fewer activities, which gives a better overview of the project, but in the same time, still goes into the level of detail with the activity group in the hammock. The bar of the hammock activity begins when the earliest activity in the group starts and end when the latest activity in the group ends, see figure 2. Figure 2: How to create a Gantt Chart. Reproduced from: "Smartsheet" [2] Step 3 - Revise til Gantt Chart: Have a look at the Gantt Chart again. Are all the important activities there and are they logically placed in the chart? What about the timescale? Are the durations reasonable? Make the necessary changes in the chart.

Critical Path Method (CPM)

The Critical Path Method was developed in the 1950s by Morgan R. Walker and James E. Kelley. CPM is very similar to PERT (Program evaluation and review techniques), but represent information differently. The CPM technique translates the project's critical tasks into a mathematical system and enables to calculate the time and resources needed to complete a project.

Definition "A schedule network analysis technique used to determine the amount of scheduling flexibility (the amount of float) on various logical network paths in the project schedule network, and to determine the minimum total project duration. Early start and finish dates are calculated by means of a forward pass, using a specified start date. Late start and finish dates are calculated by means of a backward pass, starting from a specified completion date, which sometimes is the project early finish date determined during the forward pass calculation.”

How to create the Critical path

Creating the Critical path step by step

Step 1 - Defining each activity: All activities need to be specified. The activities are usually drawn from the work breakdown structure. Step 2 - Defining activity interdependencies: The understanding of the dependencies of the activities is important when creating a critical path method. Therefore, make a table describing which activities are affecting which. Step 3 - Creating a Network Diagram: There are two techniques ways to construct a network diagram. It can either be represented by an Arrow Diagram or a Precedence Diagram. Both techniques include the basic elements of the activities that need to be performed. Like in the Gantt chart each activity is given a duration within a time limit to complete the task. The network can like in the Gantt chart also be divided into sub-networks, which are the smallest unit that can be processed and can be linked into larger networks. Figure 3: Arrow diagram. Reproduced from "CPM Scheduling for Construction - Best Practices and Guidelines" Arrow diagram Figure 3 shows how an arrow diagram can be presented. The circles represent the events and are connected to arrows which indicate the activities of the project. The time period of each activity is also indicated on the arrow. Each event represents the activity to start and finish. An event is finished when its preceding activities are done. It is important to be accurate when creating the arrow diagram because it is the base of the analysis that is performed. The order of the activities must also be placed logically. In figure 3 Building Section cannot be performed before Prepare Site has finished. The activities in the project may affect the way the network is worked out in the end. The different activities types are described below: Dummy Activities - this activity is not an actual task, but more a link between the networks paths, read more in the section Event types and activity. Ladder Activities - are activities which describe the overlapping activities in the project, read more in the section Event types and activity. Hammock Activities - as described in the Gantt chart, hammock activities can be used to summarised multiple activities in a project, which is an excellent way to make the project more manageable. Figure 4: Precedence diagram. Reproduced from: Figure 5: Dependency types. Received from "CPM Scheduling for Construction - Best Practices and Guidelines" Precedence Diagram Figure 4 shows an example of how a precedence diagram can be presented. The boxes represent here the activities in the network, and the arrows/lines are the dependencies in the precedence network. As the arrow diagram, an activity cannot start before finishing the one before, but two or more parallel activities can be worked out the same time, this is also the case in the arrow diagram. In the precedence diagram the activities are identified as A1, A2, A3 etc. see figure 5. The different activities durations is depended on the time that is required to perform the task. The dependencies in the presence network are generally more flexible than the arrow network. The dependencies are divided into four types, see figure 5: - Finish-to-start - Finish-to-finish - Start-to-start - Start-to-finish Like in the arrows diagram, it is important to have an accurate representation of the project. The dependencies are also important to ensure that the placement of the activities is logical. Step 4 - Estimating activity durations and resources: Here is the duration of each activity estimated, and listed on the network diagram. There exist different time estimation methods in network-based project management, and it is up to the individual which method is used. Step 5 - Basic Calculations: Here some basic calculation must be done, based on the expected duration of the project, the earliest start and finish dates for all activities in the project, but also the latest start and finish dates for the activities that do not interfere with the project completion time. Step 6 - Advanced Calculations - Identifying the Critical Path: The critical path is the longest route to the trough the network. It is critical because, in the end, it can delay the whole project, and give major problems. After the basic calculations usually the time cost trade-off procedures and/or resource connected calculations are done. The question about how the project manager should choose the activity durations to accomplish the lowest possible cost in the project is essential in this step. Many mathematical models handle this problem, but this article will not go into details with this. Step 7 - Control and update: It is important to update the plan when new circumstances arise. In the control phase, the project cost is measured and compared with the original cash flow provisions. Step 8 - Review: It is time to review the progress of the project and ask how accurate was the activity list, the time and resource estimation? Did the control phase help and were there any changes during the project and what were the reasons?

Event types and activity

Each activity in the project may have a set of properties which in the end effects the network whether the network is represented by an arrow or perceived diagram. These properties are offend called types and are:
- Dummy activities
- Hammock activities

For arrow network only, is there also types like:
- Ladder activities
- All event types

For precedence networks only:
- Start activities
- End activities

Dummy activities

Figure 6: Dummy activities. Received from "CPM Scheduling for Construction - Best Practices and Guidelines"

A dummy activity is not an actual task, but a logical link between the networks paths. A broken line or arrow normally indicate dummy activities. Dummy activities can be used to avoid double activities, reduce the number of dependencies and to provide a common start and finish date. Dummy activities normally have zero duration since they only represent the logical link between the networks paths and not a task. Some dummy activities do though. For example in a building project where you have wait for the concrete to cures. The time here is important in the network, and you have to indicate the duration of these activities. Such activities are called real dummies.

Ladder activities

Figure 7: Ladder activities. Received from "CPM Scheduling for Construction - Best Practices and Guidelines"

In CPM overlapping cannot be modelled in an adequate way. In network diagram overlapping cannot be modelled in an adequate way. Therefore ladder diagrams can be used to get the same early start and finish times as they can in a Gantt chart. Because the ladder diagrams cannot be interpreted in the same way as a network diagram, as they are not entirely consistent with the traditional arrow diagrams, these diagrams should be made beside the network diagram.

The Importance of the Gantt Chart and the Critical Path for Project Management

How to connect Gantt chart and CPM

The Gannt chart is useful when showing the dependencies. It is easy to produce, and update, and generally is a useful "all-around project planning" tool. For more complex, time-critical projects, the CPM is a useful tool that provides a clear indication of the critical sequences of tasks essential to keeping the project on schedule.

Pros and cons

	Pros	Cons
Gantt chart	Strong visual communication and scheduling tool An easy way to schedule all tasks and track them in the project progress next to the deadline(s) Easy to show variance from the previously issued schedule Readable for the stakeholders.	Does not show the dependency of the tasks to each other Does not show which tasks are ‘critical’ to finish the project on time. Constant updates of the chart Does not shows enough details about the subactivities that must be performed to accomplish each primary activity.
CPM	Identifying the critical activities and the critical path Graphic interpretation Can be computerised Can clarify overlapping activities in the project	Creating a network diagram can take time Difficult to show the status of the project, because of lack of a timeframe Can become unwieldy, and result in that no one uses it

Annotated Bibliography

Carson, Christopher Oakander, Peter Relyea, Craig. (2014). CPM Scheduling for Construction - Best Practices and Guidelines. Project Management Institute, Inc. (PMI)

Describes into detail the process of schedule design. The book is an excellent structured and has guidelines of how to construct, for example, Gantt Chart and CPM.

Guide, A. (2008). Project management body of knowledge (pmbok® guide). In the Project Management Institute.

A good basis for understanding the different terms and tools in project management.

Hajdu, M. (1997). Network Scheduling Techniques for Construction Project Management. Springer US.

Even so, the book is from 1997, it gives a good and detailed overview of the steps of CPM. If you want a more detailed description of the eight steps on how to create the CPM.

Milosevic, D. Z., & Martinelli, R. J. (2016). Project management toolbox: tools and techniques for the practicing project manager. John Wiley & Sons.

Provides a step-by-step guide of when and how to construct a Gantt chart, it also provides real-world examples.