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Engineering Projects - The critical path

Updated: Jul 12, 2022

Following on from our last article on engineering projects - planning for success (catch up here), this article expands on project management to discuss the importance to any project manager of knowing how to find the project critical path, and why it is so useful.

Critical path analysis is a technique for planning and scheduling a project. Let’s take a simplified project for designing and manufacturing a bicycle. Some of the main project activities can be shown in a network diagram as per below.

Critical path flow-chart
Critical path flow-chart

Notice how the project runs from left to right, and some activities can be done in parallel (e.g. making the frames, and fabricating the wheels) whilst some activities cannot start until the previous ones have finished (e.g. purchase of parts cannot start until the Design activity has finished).

Our role as the project manager is to understand which activity must start first, how much slack or ‘float’ each activity has, and ultimately determine the critical path for this project.

So, the first job to do is to add how long each activity will take (the task duration), this has been added in as the number of days in the top centre of each box:

(I suggest you copy out the below diagram and work along with the rest of this article).

Critical path flow-chart
Critical path flow-chart

Earliest Start Time (EST) - The next piece of information we will add to the diagram is something called the Earliest Start Time (EST). This details the earliest possible time that each task can start, given the duration of the tasks that must have already been completed. This is shown on the top left of each box in our diagram below.

Earliest Finish Time (EFT) - Similarly, the Earliest Finish Time is simply the earliest time each task can be finished given its start time and the duration to complete the task.

It can be written as EFT = EST + Task Duration. This number is written in the top right corner of each box.

Assuming we start the project on ‘day 0’, our network diagram would now look like this:

Critical path flow-chart
Critical path flow-chart

Notice how the earliest start time for each activity is the same as the highest finish time for the previous activities. E.g. The bicycle assembly cannot start until day 16 as this is the latest finish time of the 3 tasks that feed into it.

Doing this ‘forward’ pass of working left to right we have determined our earliest possible project finish time, 23 days.

Backwards Pass

Now, let’s consider we work backwards from our earliest project finish time. The purpose for doing this is to identify any slack within the activities, i.e. some of these activities could start later than the earliest start time without affecting the overall project.

Latest Finish Time (LFT) - The latest finish time is the latest that each task could finish without affecting the overall project.