The Critical Path Method (CPM) in Project Management

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According to the Project Management Institute (PMI), project management is the application of knowledge, skills, tools and techniques to project activities in order to meet project requirements and objectives. The challenging task of managing projects can be supported by an operation reserach technique called the Critical Path Method (CPM). The CPM is a mathematically-network based algorithm in which can be used for planning, scheduling and monitoring project progress.

The technique developed in late 1950s uses information from a work breakdowm structure (WBS) in a network representation to display interrelationships and dependencies between project activities that must be accomplished to complete a project. The technique is used for analyzing projects by determining the longest sequence of tasks through a project network, called the critical path. This determines the shortest possible time to complete the project as well as which tasks or activities should be of extra focus in the project. Furthermore, based on acquired information from the CPM, the next question could be if it is possible to shorten the project in order to finish within certain deadline.

The article includes brief introduction and background of the CPM method in project management together with the methodology behind it. An example of the method is carried out step-by-step and a solution is presented. Following is a discussion on time-cost trade offs in project acceleration together with an example of project crashing. Lastly, the article ends on a brief summary on the benefits and limitation of the CPM method within the field of project management.

Overview

Introduction

Organizations across the world and within different sectors have been using project management as a way to improve project results. The increased awareness and acceptance indicates that the application of knowledge, skills, tools and techniques can have significant impact on project success.

Figure 1: Project Management Iron Triangle

According to the ISO 21500 a project is defined as a “unique set of processes consisting of coordinated and controlled activities with start and end dates, performed to achieve project objectives. Achievement of the project objectives requires the provision of deliverables conforming to specific requirements.”

Each and every project is carried out under certain constraints. The most traditional ones, the three constraints of cost, time and scope represent together the iron triangle of project management, which can be seen in Figure 1. Each constraint forms the vertices with quality as the central theme and all together it indicates that projects must be delivered within agreed time and cost and furthermore to meet the agreed scope and customer quality requirements. The three constraints are closely linked and changes to one of them will likely affect the others, or impact the quality of the project. This emphasizes the importance of project management and that the area is challenging an not at all easy to deal with.

Project management is accomplished through the appropriate application of five processes:

1. Initiating
2. Planning
3. Executing
4. Monitoring and Controlling
5. Closing

Each and every organization can be involved in a project of any size, duration and complexity level at any given time. It is notable that project planning, scheduling and monitoring is a major part involved in project management. Therefore the critical path method (CPM) can be of good support within the challenging process of managing projects and helps those involved to address questions such as:

• What is the (minimum) total time required to complete the whole project?
• What are the (earliest and latest) start and completion times for individual activities?
• Which activities are critical and must be completed on time in order to complete the whole project on time?
• How much delay is tolerated of non-critical activities without impacting the overall project schedule and project completion time?
• What is the least expensive way to speed up a project to meet a targeted completion time?

All these questions are valid and important to have under control when managing projects.

Background

The critical path method (CPM) is a mathematically-network based project modeling technique developed in late 1950s in order to plan, schedule and control large, complex projects with many activities. CPM is one approach of network techniques that has been widely used and was developed by a DuPont engineer Morgan R. Walker and a Remington Rand computer expert, James E. Kelly, Jr. to manage plant maintenance and construction work. Furthermore, a critical path network technique developed simultaneously and independently from CPM, called PERT (Program evaluation and review technique). PERT was developed by the U.S. Navy for managing Fleet Ballistic Missile (Polaris) submarine project and is commonly used in conjunction with CPM and and the two optimization techniques are ofen referred to collectively. Today, there exists project management software packages in which are based on these techniques, eg. Microsoft Projects.

The CPM technique has been used for many different forms of projects including construction of a new building, bridge or a road, construction of IT-systems, building of ships and research and development of new products. All these projects consist of multiple activities or tasks that have to be completed in order to finish the whole project. Furthermore, some activities are independent and therefore cannot start before other activity has been completed and so on. The ones who manage projects have to deal with multiple challenges within planning, coordinating and monitoring these activities in order to complete the project of interest successfully within agreed time, cost and scope and according to customer quality requirements.

Application and use

Methodology

The following input requirements are needed in order to construct a network model of the project of interest:

• A list of all activities or specific tasks required to comlete the project also known as the work breakdowm structure (WBS)
• The duration of each activity for completion
• The dependencies between the activities

With this information, the critical path method is able to determine the longest path of activities through the project network, defined as the critical path and the corresponding activities are defined as critical activities. Based on the fact that the entire project cannot finish until all the activities are completed, the longest path or the critical path gives also the minimum required time to complete the entire project.

Figure 2: Project Network Legend

The critical path is of great importance since all critical activities must be completed as scheduled in order to meet the scheduled project completion time, that is, if an activity on the critical path is delayed, the project is delayed by the same amount of time. This is due to the fact that the activities on the critical path have a zero slack. Slack (SL) is defined as the amount of time an activity can be delayed without delaying the whole project. Therefore, project managers need pay close attention to the critical path activities in order to make sure that they are not delayed.

The sensitivity of a project network is a function of the number of critical or non-critical paths. A network is considered sensitive when it has more than one critical path and/or non-critical activities with little slack and therefore, it is considered more likely that the original critical path(s) will change, once the project has started. On the other hand, a network is considered insensitive when it has one critical path and non-critical activities with very large slack.

• ID: Identification code of the activity
• DUR: Duration time of individual activity
• ES:
• EF
• LS
• LF
• SL

Consdideration of Time-Cost trade-offs

Project Crashing Example

Once the critical path and timeing of activities has been identified - the next question is if it is possible to shorten the project in order to finish within a certein deadline.

Here is maybe possible to do an example of project crashing.

Benefits of CPM

1. Benefits in planning, sceaduling and monitoring