1. Activity identification.
The first step in CPM is to identify and list all the activities required to complete the project. This is typically done through a Work Breakdown Structure (WBS) — a hierarchical breakdown of the total scope into smaller, manageable tasks. The WBS ensures that all necessary work is accounted for and provides a clear foundation for scheduling and tracking.
Getting the right level of detail is essential. While the WBS should be comprehensive, breaking tasks down too granularly can complicate analysis — especially for large projects. Focus initially on major deliverables and key activities to maintain clarity and manageability.
2. Establish task sequence.
Once all activities are identified, the next crucial step is to determine the logical order in which they must be performed. This involves identifying dependencies between tasks, specifically:
- Predecessor activities: Tasks that must be completed before another specific task can begin.
- Successor activities: Tasks that can only begin after one or more specific tasks have been completed.
Understanding these relationships is fundamental to constructing an accurate project network. There are several types of task dependencies:
- Finish-to-Start (FS): The most common type of dependency. Task B cannot start until Task A is finished.
- Start-to-Start (SS): Task B cannot start until Task A has started. These tasks may run in parallel.
- Finish-to-Finish (FF): Task B cannot be completed until Task A is finished.
- Start-to-Finish (SF): A rarer dependency type. Task B cannot finish until Task A has started.
These dependencies can also be categorized based on their nature:
- Mandatory dependencies: These are inherent in the work and cannot be avoided. An example is that a foundation must be laid before walls can be built.
- Discretionary dependencies: These are defined by the project team based on best practices, experience, or specific preferences. An example includes painting a room after all the electrical work is done.
- External Dependencies: These involve relationships between project activities and non-project activities, often outside the direct control of the project team. An example includes waiting for regulatory approval before proceeding with a particular phase.
3. Create a network diagram.
With activities and their dependencies defined, the next step is to create a project network diagram. This is a visual, flowchart-style representation of the project's tasks and their interdependencies — illustrating the workflow from start to finish. The network diagram is crucial for understanding the overall project structure, identifying various paths through the project, and forming the basis for critical path calculations.
There are two primary methods for drawing network diagrams:
- Activity-on-Node (AON) or Precedence Diagram Method (PDM): Tasks are represented by nodes (typically boxes or circles), and arrows are used to show the logical dependencies between these tasks. AON is the most used method in modern project management due to its simplicity and ability to represent all types of dependencies directly. This approach evolved to provide greater clarity and ease of use compared to older methods.
- Activity-on-Arrow (AOA): Activities are represented by arrows, and nodes (circles) represent events or milestones marking the start or end of activities. Dotted lines represent dummy activities in AOA diagrams and have a duration of zero; they are used to maintain correct logical sequencing or to separate tasks that might otherwise share the same start and end events.
4. Estimate activity completion time.
Assigning an estimated duration to each identified activity is a critical input for the CPM process. The accuracy of these duration estimates directly impacts the reliability of the critical path and the overall project schedule. Inaccurate estimates are a primary reason why CPM implementations fail, as they lead to a flawed critical path and misleading float calculations — undermining all subsequent planning and control efforts.
Several techniques can be employed for estimating activity durations:
- Expert judgment: Relying on the experience and knowledge of team members or subject matter experts who have performed similar tasks.
- Analogous estimating: Using historical data from similar past projects or tasks as a basis for estimating the durations of current tasks. This is often quicker but may be less accurate if the projects are not truly comparable.
- Parametric estimating: Using a statistical relationship between historical data and other variables to calculate durations. This technique can be highly accurate if the underlying data and model are reliable.
5. Identify the critical path.
And finally, you need to identify the parts of the project that must be completed — and in what order. The critical path is the longest possible sequence of events providing a time range for each task and the entire project. By identifying which time-consuming tasks are critical to project completion, teams can work diligently to complete those tasks or recognize that their end date will be delayed.
