The critical path method (CPM) is a project management scheduling method. It defines the sequence in which specific activities are carried out to complete a project within a given time frame and budget.
Action in the critical path cannot be delayed or shortened in duration, for the whole project to finish on time. On the other hand, the critical chain method (CCM) is a form of schedule network analysis that adds activity between predecessors and successor activities. The additional activity allows a set of actions to be completed as one extensive activity called a milestone, enabling these activities’ completion date to be slack.
What is a Critical Path?
A critical path is the path in a network diagram that entails various paths consisting of several exercises linked by specific types of relationships. All the practices on a critical path are referred to as critical activities.
A delay in these activities delays the tasks in a critical path with the same length of time. The period the project takes is the length of the critical path
Steps to Draw a Critical Path Diagram
1. Specify Each Activity
A Project manager has to use a work breakdown structure (WBS) to specify each activity involved in the project. The specification should only focus on higher magnitude activities. Detailed activities may make the critical path very complex thus tricky to maintain and manage.
The work breaks down structure reduces the activities into sizeable and manageable sections. To use a WBS, a project manager needs to first identify the core deliverables of a project. From this, break down the higher magnitude activities into sizeable chunks of work. An outline is the most preferred method to highlight a work break sown structure.
2. Establish Dependencies
In the critical path, some activities do rely on the completion of another activity. Therefore, a project manager needs to list every action’s intermediate predecessors to get the correct order. To accurately determine these activities and their intermediate predecessors, a project manager needs to answer these questions:
- Which task should come before a particular job?
- Which tasks should be completed at the same time?
- Which job should follow immediately another job?
3. Draw the Network Diagram
A project manager can draw the critical path analysis chat (CPA) from the activities and the network diagram’s dependencies. The critical path analysis chat is the visual view of the activities’ arrangement with their dependencies. There is a variety of software which a project manager can use to draw this diagram.
4. Estimate Completion Time for an Activity
A project manager needs to incorporate the knowledge on past projects related to the current and estimate the time the activities will take. He can estimate the time for shorter projects in days while that for more extended projects in weeks or months depending on the project’s complexity.
In a scenario where the manager doesn’t trust the best guess estimate of similar projects, they can use a 3-point estimation method. The 3-point estimation method exerts more weight on the most realistic timeframe. The 3- point estimate requires a project manager to develop three consecutive time estimate for each task, basing on the best guess or previous experience. And it follows the formula:
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- a = the best-case estimate
- m = the most likely estimate
- b = the worst-case scenario
The project manager then uses these values in two unique procedures. The first formula targets getting the weighted average, which focuses on the most likely value, as shown below.
- E refers to the estimate
Numbers 4 and 6 above stand for the standard method.
The second formula is using the triangular distribution method. This method, unlike the first, doesn’t value the most likely. The formula is thus:
- E= (a+ m + b)/3
E refers to the estimate while 3 is the standard method
5. Identify the Critical Method
There are two significant ways of identifying the critical path: locating the longest path throughout the entire network. The longest route is the most extended series of activities on the path. Ensure to check the longest path while considering the length in days and not basing on the path with more nodes or boxes.
The second way is highlighting the critical activities using the forward pass? The backward pass technique highlights the earliest start /finish times and the latest start and ends each action time.
A company that has multiple critical paths is likely to run into a network sensitivity. Network sensitivity is where the critical path has higher chances of changing once the project starts. The higher the number of critical paths of the project, the probability of changing the schedule.
6. Update the Critical Path Diagram to Show the Progress
The completion time of the activity becomes evident as the project progresses. During this period, the manager can update the details on the critical path and avoid using the estimations. The new details on the critical path make it easy to calculate the critical path, and the manager can have a clear and exact view of the project completion time and evaluate the chances of losing track.
Various Types of Floats in the Critical Path
Afloat refers to the duration an activity, project, or network path is delayed from the beginning without modifying the project’s end date. The significant types of float in a critical path include:
- Total float
Total float refers to the duration in which an activity is delayed without delaying the whole schedule and is zero on a critical path
- Free float
Refers to the duration in which a process is delayed without delaying the early beginning of its successor activity.
What is a Critical Chain?
Critical chain refers to the longest path in the network diagram with resource constraints and activity interdependence. The critical chain method is an advance of the critical path such that a project manager considers resource availability in developing the project schedule. The critical chain method incorporates buffers rather than floats. The buffers get rid of the concept of slack or float. The buffers in the critical chain include:
- Project Buffer
Project buffers serve as the contingency of the critical chain activities and are typically incorporated between the final task and the project end date end as a non-activity buffer. The gain from activities that end earlier than the expected completion time is placed in this buffer.
The duration of activities is usually 50% of the contingency a project manager removes from each task. It helps eliminate the uncertainties from a job to the project buffer, enhances efficiency, and lowers the schedule duration.
- Feeding Buffer
Feeding buffer is incorporated into non-critical chain such that delays that occur in the critical chain don’t alter with the critical chain. The calculation of project and feeding buffer is the same, and the lifecycle of these buffers relies on the safety that a project manager removes from the tasks on a non-critical chain.
- Resource Buffers
They are kept along with the critical chain to enhance their availability when needed, and they can be pieces of equipment or human resources. A critical chain’s lifecycle will be greater than that of a critical path since they consider the resource constraints.
The duration can, however, be reduced when a project manager eliminates the contingencies from the activities. The critical chain makes use of critical resources to display its data.
How to Create a Critical Chain Network Diagram?
The first step in creating a critical chain diagram is developing a critical path diagram. After that, the project manager should follow the following steps:
- Eliminate all the contingencies from activities.
Substitute the estimate with a realistic assessment if you used Program Evaluation and Review Technique (PERT) calculation to establish the schedule.
- Eliminate the resource constraints and arrange the activities together with late finish dates.
The critical chain activities are given more preference when allocating resources.
- Non-critical chain
In the case of a non-critical chain, the project manager should add the feeding buffers to have the same lifecycle as the critical chain. He should add the project buffer at the end of the critical chain before the project.
Differences Between Critical Path and Critical Chain
The significant differences between critical path and critical chain are:
- The critical path focuses on managing activities, whereas the critical chain concentrates on managing the buffer.
- The critical path assumes that all the resources will be readily available when needed. In contrast, the critical chain takes that resources limited and uses the available resources to construct a realistic schedule.
- The critical path doesn’t appreciate completing a project earlier since the next activity can’t begin before its early start time. On the other hand, the critical chain enjoys the time since when an activity ends earlier, you can start the following action, and the gained time is incorporated in the buffer.
- The team members in critical path can’t begin to activity until its late start time when it has afloat. The members, therefore, have to utilize the float.
- Critical paths give an allowance to add the delay but not the gain. The work increases if an activity has extra time, which isn’t the case in a critical chain since every activity has its real-time length, and the buffer is incorporated at the end of a project.
Examples /Application of Critical Path and Critical Chain
An electrical engineer was allocated a project to conduct the wiring process of the company. He developed the schedule basing on the critical path method and began his work.
However, while he was still launching his project, he realized that the wires were not enough. The equipment and materials he needed were also assigned to other projects. The company had taken two members for some urgent work.
For this project, the issue was with the allocation of resources. The critical path recognized the resources but didn’t include the scarcity of resources. The engineer established his schedule to assume that the resources will be readily available whenever he needs them, yet this wasn’t the case. The program was then delayed.
To manage this problem, the engineer had to use resource constraints which transformed the critical path to the critical chain, which is more realistic and exact. He, therefore, completed his project with no difficulties.
- Highlights the most relevant tasks
critical paths indicate the functions which require much attention. Whenever an activity on the critical path finishes longer than estimated or take more time than their predicted duration, then the entire project will be affected.
- Help reduce timeliness
Critical path highlights its activities making it easy for a project manager to visualize them. They thus give an insight into the timeline of a particular project. Hence the manager can quickly get a view of the projects which require adjustments in their duration and those which remain the same.
- Compares planned progress with actual progress
While using critical paths, a project manager needs to develop a baseline schedule at the beginning of a project. The baseline schedule helps track the schedule progress .as the project continues, the project manager can recognize the tasks that have been finished, the remaining time for the ongoing project, and the changes for future assignments. The results are usually an updated system which, when displayed against the actual baseline, offers a visual means of comparing actual with planned progress.
- There are no unlimited project resources
- The buffers allow adding the risks which may delay the activities on the critical chain
- Any delays in any critical activity may delay the project
- It assumes an unlimited supply of resources
- There are low possibilities of finishing the project within the predicted time
- The method relies on the project manager, stakeholders, and team members hence delays, when some of the members don’t know their roles
- Only highlights the duration of time as the significant factor affecting completion of a project yet internal politics and external pressures also affect the duration
The critical chain and critical path methods are essential tools every organization needs to have. They offer assessments of actual time for a planned activity for the project managers to evaluate their project’s progress. The critical chain and critical path also give a project manager the chance to determine the activities that take more time than expected, those ahead of schedule, and those right on the track.