A Guide to The Critical Path Method (CPM)

What is the Critical Path?

The Critical Path Method (CPM) enables the project Critical Path to be determined. This is a sequence of activities through a project network from start to finish. There may be more than one Critical Path depending on workflow logic. A delay to progress of any activity on the Critical Path will (without acceleration or re-sequencing) cause the overall project duration to be extended. This is known as a ‘critical delay'.

What is the Critical Path Method (CPM)?

The Critical Path Method is a mathematically based algorithm for scheduling project activities. It was developed in the 1950s in a joint venture between DuPont Corporation and Remington Rand Corporation, specifically for managing plant maintenance projects. Whilst the method was developed for construction, this method of scheduling can be applied to any project with interdependent activities.

The Critical Path Method, which is also sometimes referred to as Critical Path Analysis (CPA) method, deduces the critical activities in a programme by tracing the logical sequence of tasks that directly affect the completion date of the project

It is a methodology or management technique that determines a project's critical path. CPM provides the following advantages:

1. Determines the time to complete the project and key milestones

2. Allows tracking of critical activities

3. Provides a visual presentation of the project

4. Can incorporate the concepts of random predictions, using (PERT)

CPM models the activities and events of a project as a network. There are two main types of logic diagrams:

1. Arrow Diagram (AOA)

2. Precedence Diagram (AON)

Key Steps in Critical Path Method Project Planning

1. Specify activities

2. Establish Activity Sequence

3. Produce network diagram

4. Estimate the completion time for each activity

5. Identify the critical path (longest path through the network)

6. Update the CPM diagram as the project progresses

1. Specify Activities

* Based on the work breakdown structure (WBS), produce a list of all the activities in the project.

* This listing will be used as the basis for additional sequence and duration information.

2. Establish Activity Sequence

Determine the correct sequence of the tasks. Do this by asking three questions for each task:

i. Which tasks must happen before this one can begin?

ii. Which tasks can be done at the same time as this one?

iii. Which tasks should happen immediately after this one?

It can be useful to create a table with four columns:-prior tasks, this task, simultaneous tasks, following tasks.

3. Produce Network Diagram

Once the activities and their sequencing have been defined, the CPM diagram can be drawn. CPM originally was developed as an activity on node (AON) network. Now computers and planning software such as Primavera are used to produce the Network Diagram. however the principles of AON networks should be understood.

4. Determine Activity Durations

Determine a reasonable time required to complete each activity based on metrics, past experience or the estimates of knowledgeable persons. It is important to record how the estimated durations were derived and from what source of information. the Critical Path Method is a deterministic model that does not take into account variation in the completion time, so only one number is used for an activity's time estimate.

5. Identify the Critical Path

Determine the following four parameters for each activity:

i. ES - earliest start time

The earliest time at which the activity can start given that its precedent activities must be completed first.

ii. EF - earliest finish time

Equal to the earliest start time for the activity plus the time required to complete the activity.

iii. LF - latest finish time

The latest time at which the activity can be completed without delaying the project.

iv. LS - latest start time

Equal to the latest finish time minus the time required to complete the activity.

The Float (slack time) for an activity is the time between its earliest and latest start time, or between its earliest and latest finish time. Float is the amount of time that an activity can be delayed past its earliest start or earliest finish without delaying the project.

The critical path is the longest-duration path through the network. The activities that lie on the critical path cannot be delayed without delaying the project. The critical path is the path through the project network in which none of the activities have float, that is, the path for which ES=LS and EF=LF for all activities in the path.

To accelerate the project it is necessary to reduce the total time required for the activities in the critical path.

6. Update the CPM diagram

Update the CPM diagram as the project progresses. The task completion times will be known and the network diagram can be updated to include this information. A new critical path can emerge, and structural changes may be made in the network if project requirements change. This is a key element in managing the project and fundamental aspect of Change Management.

[Modified from NetMBA Business Knowldege Centre, NetMBA.com]

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