Critical path method: How to use CPM for project management

Finding the critical path involves looking at the duration of critical and non-critical tasks:

CPM steps:

1. List activities
2. Identify dependencies
3. Create a network diagram
4. Estimate task duration
5. Calculate the critical path
6. Calculate the float

List all the project activities or tasks required to produce the deliverables of each task.

The list of activities in the work breakdown structure serves as the foundation for the rest of the CPM. 

From your work breakdown structure, determine which tasks are dependent on one another to identify any work that can be done in parallel with other tasks. The dependent tasks, known as an activity sequence, will be used to determine the critical path.

For example, a marketing team is producing a new interactive blog post. In the example above:

• Task B is dependent on A
• Task C is dependent on B
• Tasks C and D can run in parallel
• Task E is dependent on D
• Task F is dependent on C, D, and E.

A network diagram is a flowchart that shows the chronology of activities.

Create a box for each task and use arrows to show task dependencies. Add other time-bound components to the network diagram until a general project schedule is worked out.

To calculate the longest sequence of critical tasks, you need to estimate the duration of each activity.

How to estimate the duration:

• Make educated guesses based on previous experience and knowledge.
• Estimate based on previous project data.
• Estimate based on industry standards.

Use the forward pass and backward pass technique to calculate float.

• Forward pass is used to find early start (ES) and early finish (EF) dates by using a previously specified start date. EF is ES + duration.
• Backward pass is used to calculate late start (LS) and late finish (LF) dates. LS is LF -duration.

Calculating the critical path can be done manually, but a critical path algorithm can save time.

Step 1: Write down the start and end time next to each activity. The first activity time is 0 and the end time is the duration of the activity. The next acitity start at the previous activity end time. Do this for all activities.

Step 2: Look at the end time of the last activity in the sequence to determine the duration of the entire sequence. 

Step 3: The sequence of activities with the longest duration is the critical path.

Float, or slack, refers to a given task's flexibility. It shows how much the task can be delayed without impacting subsequent tasks or the end date of the project. Critical tasks have zero float. Their dates are set.

How to determine the two types of float:

• Total float is the time an activity can be delayed without impacting the project finish date. Total float = LS - ES or LF - EF .
• Free float is how long an activity can be delayed without impacting the following activity. Free float = ES (next task) - EF (current task)

• CPM provides visibility into your project’s progress.
• At times, a project deadline may e pushed up with fast-tracking (parallel processes speed up overall duration) and crashing (allocating more resources to speed up activities.)
• Resolve resource shortages. When there is a resource shortage, like an overbooked team member or lack of equipment, you can use resource levelling techniques to solve the issue. 
• Compile data for future use.