The critical path is the longest sequence of activities in a project plan which must be completed on time for the project to complete on due date. An activity on the critical path cannot be started until its predecessor activity is complete; if it is delayed for a day, the entire project will be delayed for a day unless the activity following the delayed activity is completed a day earlier.
The critical path is very useful in helping to manage any project. When the critical path has been identified, it can clearly be seen where effort cannot be compromised. If any of the activities on the critical path change, the end date of the project will be affected.
Critical Path Analysis
The work needed to complete the project needs to be broken down and all activities need to be defined. Once we know how long each activity will take, we can use this information to understand the duration of the project. A network schedule of activities needs to be completed. Each activity within the schedule is represented as follows:
Early Start – The earliest time that an activity can start according to the logical constraints.
Duration – The estimated time to undertake the activity.
Early Finish – The earliest time that an activity can finish according to logical constraints.
Late Start – The latest time that an activity can start according to logical constraints and without affecting the overall project duration.
Float – The time by which an activity may be delayed without affecting the overall project duration.
Late Finish – The latest time that an activity can finish according to logical constraints and without affecting the overall project duration.
In order to work out how long it will take to complete the sequenced work, we need to perform what is known as a forward pass. The early start of the first activity is zero and the early finish is calculated by adding the duration.
Early Start + Duration = Early Finish
This process is then carried through subsequent activities. Where an activity has two or more preceding activities it is the latest time which is transferred.
(You can click on the critical path diagrams to make them larger).
The example shown indicates that the quickest we can carry out the work we have identified is 28 days. However, what we don’t know is which of the activities are critical and if we have movement available (float) on any of the activities.
In order to establish the latest dates that an activity can commence without affecting the end date a back pass is performed. The early finish of the last activity in the network is transferred to the late finish. The duration is then subtracted from the late finish to obtain a late start. Where an activity has two or more succeeding activities, it is the earliest date that is transferred. This process is repeated throughout the network until all late start and finish dates have been identified.
Late Finish – Duration = Late Start
We can now work out what flexibility or float we have in the network. This is very important to the project manager as it will allow for decisions to be taken with the allocation of resources to maximise their utilisation.
There are two types of float:
Free Float – this is the amount of time a task can be delayed without affecting the succeeding task. This can be calculated by subtracting the Early Finish of an activity from the Early Start of its subsequent activity.
Total Float – this is the amount of time which an activity can be delayed without affecting the end date of the project.
Latest Finish – Earliest Finish = Float
Now that we have the early start and late start for each activity and have calculated the float available, we can work out the critical path through the network. The critical path is the series of activities within the network with zero total float. The critical path is shown in red in our example given.
There must be at least one critical path through any network. The path must be continuous but it may branch into a number of parallel paths.
It is important to remember that a critical path can change during a project, as actual durations and dates vary.
A helpful technique for controlling and managing the critical path is to invest some time in determining what is likely to go wrong in each of the main three project parameters, ie cost, time, quality. Although the critical path only reflects the time element, a compromise in either cost or quality can have a time impact.