Using critical path analysis to catch your plane

Category: Coaching Notes

My first degree was in civil engineering, including a final year elective on project management. In this subject I was introduced to critical path analysis. Many years later I read The Goal, by Eliyahu M Goldratt. It is based on his theory of constraints, a similar principle of ensuring that work can flow unhindered by bottlenecks and delays.

Since then I have encountered various frustrations in which other people seem to miss an obvious point – such as booking my air ticket early. For example, I know that not only do airfares increase as the date of departure gets nearer but that when all the seats have been sold it is no longer possible to book a flight! If getting to a destination is critical then without a flight you can have a serious problem. In a lesser case, the longer you delay your booking, the more it will cost you. And yet I have had difficulty in getting support staff to give it high priority.

Booking your flight is critical! It is a simple example. It might be more complicated if before you can commit to a date you need other people to make decisions, or you need to obtain various letters or certificates in order to apply for a visa. The various stages can be laid out on a critical path to ensure that each is done in time.

In this case there is likely to be only a single path and, by working backwards from the end point, you can easily decide when each activity must be completed. More complicated projects will have a network of activities which rapidly become too difficult to assess by eye. This is where critical path analysis becomes useful. First you draw up the network of activities, deciding what activities need to be completed before others can be started. Then you calculate the earliest and latest start and finish times and identify the critical path through the network.

The Critical Path Method

The Critical Path Method (CPM) models the activities and events of a project as a network. Activities are depicted as nodes on the network and events that signify the beginning or ending of activities are depicted as arcs or lines between the nodes.

CPM provides the following benefits:

Provides a graphical view of the project.

Predicts the time required to complete the project.

Shows which activities are critical to maintaining the schedule and which are not.

The critical path is the longest-duration path through the network. The significance of the critical path is that the activities that lie on it cannot be delayed without delaying the project. Because of its impact on the entire project, critical path analysis is an important aspect of project planning.

Finding the critical path

The critical path can be identified by determining the following four parameters for each activity:

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

EF - earliest finish time, equal to the earliest start time for the activity plus the time required to complete the activity.

LF - latest finish time: the latest time at which the activity can be completed without delaying the project.

LS - latest start time, equal to the latest finish time minus the time required to complete the activity.

The slack time for an activity is the time between its earliest and latest start time, or between its earliest and latest finish time. Slack 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 path through the project network in which none of the activities have slack, that is, the path for which ES = LS and EF = LF for all activities in the path. A delay in the critical path delays the project. Similarly, to accelerate the project it is necessary to reduce the total time required for the activities in the critical path.

Steps in CPM Project Planning

Specify the individual activities.

Determine the sequence of those activities.

Draw a network diagram.

Estimate the completion time for each activity.

Identify the critical path (longest path through the network)

Update the CPM diagram as the project progresses.

Like many of the processes that Brefi Group teaches, one of the advantages of CPM is that after some practice it becomes possible to visualise simple critical path networks in one's head – and more easily recognise the critical activities like booking the air tickets and hotel.

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