Critical Path Method
The critical path method (CPM) is the most popular scheduling method. Even though it is a relatively old method, project managers still use it. It is true that agile teams no longer use it. Still, CPM is often part of the scheduling solution for conventional, iterative, incremental, and hybrid teams.
The Critical Path Method (CPM) determines the start and finish dates for each activity. Moreover, it determines the project duration considering its critical path. In other words, it schedules the project considering the path with the least schedule flexibility. In this post, we will explore these concepts in depth. You may also want to check these posts to learn more about project management and project schedules.
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Critical Path Method (CPM) Overview
Projects involve various activities that must be completed in a specific order or sequence to deliver the product. For example, you have to dig a hole before planting a tree. Moreover, different activities take a different amount of time and resources. Digging a hold, for example, may take longer than planting a tree. Additionally, several activities may be done concurrently. For example, in the diagram below, several activities occur concurrently.
The Critical Path Method (CPM), also known as Critical Path Analysis (CPA), represents activities and dependencies. Moreover, it highlights the activities that cannot be delayed without delaying the project. In other words, it shows the critical path as the activities without schedule flexibility. Our example includes two sequences of activities. Firstly, the “Start-D-E-F-Finish” sequence takes 9 days. Secondly, the “Start-A-B-C-Finish” sequence takes 19 days. The second path takes longer and is, therefore, the critical path. In other words, delays in these activities have a direct impact on the project duration. Moreover, the project duration is the sum of the duration’s of activities on the critical path. Therefore, in the previous example, the project duration is 19 days.
Exploring the Critical Path Method (CPM)
Listing Activities
If you want your project to be successful, the first thing you need to do is identify the project activities. Therefore, project teams rely on the Work Breakdown Structure (WBS) to decompose work packages into activities. More specifically, they should define the actions that must be done throughout the project. So project teams should not skip this crucial step. Instead, they should take the time to use the WBS effectively. You may check this post to learn more about the Work Breakdown Structure (WBS).
Determining Dependencies
The project management team identified the activities. The next step is to assess dependencies. In other words, the project management team should prepare a network diagram. A network diagram, also known as a project schedule network diagram, visualizes the sequence of activities required to complete a project. This project management artifact relies on boxes to represent activities. Moreover, it relies on arrows to map dependencies. This diagram can, therefore, show which activities are done in which sequence. The network diagram is a dynamic artifact. It should evolve as more information becomes available. The project management team may need to add, adjust, or remove dependencies. It may also need to adjust the activities’ durations. The network diagram will later be the base for the critical path.
Estimating Durations
The project management team identified and sequenced activities. The next step is, therefore, to estimate the duration of each activity. There are several methods that the team can use to achieve this. For example, it can rely on expert judgment, analogies, benchmarks, and parametric modes. With accurate duration estimates, team members can therefore allocate resources effectively. They may also make better decisions. Don’t underestimate the importance of quality estimates to successful project completion!
Determine the Critical Path
The project management team identified, sequenced, and estimated activities. The next step is, therefore, to determine the critical path.
The following table includes an example of a set of activities, dependencies, and durations.
Determining earliest dates
Firstly, CPM determines the activities’ earliest dates. More specifically, it determines an earliest start (ES) for each activity and records it on its top left side. It also determines an earliest finish (EF) date for each activity and records it on its top right side. For example, the following image represents the network diagram with activities, durations, earliest start dates (ES), and earliest finish dates (EF). The earliest start date (ES) of activity B is 0. In other words, the earliest date that you can start activity B is on day 0.
CPM determines the EF by adding the duration to the earliest start date (EF = ES + Duration). Consequently, the earliest finish date (EF) of activity B is 5 (0 + 5 = 5). Moreover, the EF of a predecessor activity will roll forward to the successor activity. For example, the ES of activity is 5 because the EF of activity B is also 5.
When two paths converge to a single activity, the later EF of the predecessor activities will determine the ES of the successor activity. Additionally, the ES of an activity with more than one predecessor will derive from the later EF. For example, the EF of activity F determines the ES of activity F. Even though activity D is also a predecessor of activity G.
CPM repeats the process for each activity and determines their ES and EF dates.
Determining latest dates
Secondly, CPM determines the activities’ latest dates. More specifically, it determines the latest start (LS) for each activity and records it on its bottom left side. It also determines the latest finish (EF) date for each activity and records it on its bottom right side. For example, the latest start date (LS) of activity D is 80. In other words, the latest date that you can start activity D is on day 80. At the same time, the latest finish date (LF) of activity B is 90.
CPM determines the LF of a successor activity by rolling backward the LS of the successor activity. For example, the LF of activity D is 90 because the LS of activity G is also 90. Moreover, CPM determines the LS of an activity by subtracting the duration from the latest finish date (LS = LF – Duration). For example, the LS of activity D is 80 (90-10).
CPM repeats the process for each activity and determines their LS and LF dates.
The method may then determine the critical path. In other words, it can determine the sequence of activities with the longest duration.
Assessing Floats
Slack, also known as float, is the amount of time that an activity can be delayed without delaying the project. In other words, it expresses the activity’s schedule flexibility. A critical activity does not have schedule flexibility. For example, activity G is critical because its earliest dates match its latest dates.
Concurrently, some activities are not critical because they have slack. In other words, a delay in this activity does not necessarily lead to an overall schedule delay. For example, activity C is not critical because its earliest dates do not match its latest dates.
Compressing the Schedule
Projects exist within constraints that limit project decisions. For example, senior management often imposes schedule deadlines that may lead to schedule compression. In this section we will explore alternative ways to compress the schedule.
Fast tracking
Project managers can compress the schedule by changing the network diagram. More specifically, they can adjust dependencies to perform critical activities concurrently, instead of performing them sequentially. Consequently, the project manager reduces the overall project duration. However, changing dependencies will typically lead to risk. Moreover, keep in mind that the project manager can only change discretionary dependencies.
Crashing
Project managers can compress the schedule by adding resource capacity to critical activities. More specifically, they can use more resources or more skilled resources. They may also shift resources from non-critical activities to critical activities. Consequently, the project manager reduces the overall project duration. However, crashing will typically lead to a cost increase. Moreover, keep in mind that the project manager should focus on crashing critical activities. Moreover, crashing is only possible if the nature of the activity in question is not of a fixed duration. In other words, if its duration varies with resource allocation.
Renegotiate Scope and Quality Requirements
Finally, project managers can compress the schedule by negotiating scope and quality targets. More specifically, they can suggest that deliverables are removed from the scope of work. They may also suggest the removal of scope or quality requirements. However, renegotiating deliverables and requirements will influence the project’s outcome. Moreover, it will impact customer satisfaction. You may check these posts to learn more about product scope and quality requirements.
Conclusion
In summary, the critical path determines the earliest dates, latest dates, and floats for each activity. Consequently, it determines the critical path and the project duration. Project managers can rely on the CPM to assess, compress and optimize the schedule.