Introduction
The critical path of a project is the sequence of activities with zero delay margin, or float. Therefore, the critical path is the longest continuous sequence of activities in the project, starting from the last activity or milestone. A delay in any activity on the critical path results in a delay to the project completion date.
The float of an activity is calculated as late start minus early start, or late finish minus early finish for activities that have started. Negative float is a concept that refers to the amount of time that must be recovered to complete a project on time. Another way to understand negative float is the time beyond the scheduled completion date that a project or activity requires in order to finish. Therefore, for negative float to exist, there must be a constraint setting the finish date of an activity.
Sometimes the planned project activities finish before the contractual date. In those cases, it would be a mistake to define the date of the last activity using a constraint rather than relationships, because the algorithm will mark only that activity as critical.
Subcritical path
In addition to the critical path, the subcritical path must also be analysed. Although the critical path is the longest path through the schedule, attention should be paid to any task that could become a new critical path for the project if it is delayed enough. The float of activities considered subcritical depends on the type of project and on the assumptions that define activity durations and relationships.
Risk path
The risk path is considered to be the sequence of non-critical activities that, if an unexpected event occurred, would carry the greatest risk of impacting the project finish date. When analysing schedule risk in detail, it is not enough to identify the critical and subcritical paths; it is also useful to analyse whether any activity, despite having float, could be delayed enough to impact the schedule.
An example of a risk path could be the manufacture and delivery of very specific components or materials that are necessary to complete the project. A financial risk has been identified in the supplier. If the contracted supplier is finally unable to supply the product, the time required to obtain it from another supplier exceeds the time available before project completion. A risk like this must be evaluated and the progress of the subcontractor or supplier must be monitored in detail.
Primavera P6
Knowing and being able to analyse the critical path of a project is an essential part of project management. It is essential for a planner to understand how CPM software calculates the critical path in order to identify project risks.
Primavera P6 is the most widely used software for planning complex projects. It has the tools required to visualise, analyse and efficiently control the critical path, enabling more informed decision-making.
The float, or delay margin, of an activity is calculated as late start minus early start. Primavera P6 calculates late and early dates in two steps. First, early start dates are calculated through a forward pass from the first activity. Second, Primavera P6 calculates late dates through a backward pass from the last activity.
To calculate the critical path as the longest path, the last activity of the project is identified and all driving relationships are traced from the last activity back to the start date, provided that no constraint prevents the driving predecessor activity from being identified.
Once the float of an activity is defined as the difference between the late date and the early date, it is possible to understand how activities with different calendars and working hours can produce positive float. In the following example, the activity Ground floor Slab has one hour of float. The issue comes from the difference between the calendars: the 5x8 calendar works Monday to Friday from 08:00 to 17:00 with a one-hour break, while the 6x10 calendar works from 07:00 to 17:00 Monday to Saturday. Therefore, the late finish of Ground floor Slab is 14 May at 08:00 and the early finish is 13 May at 17:00.
To explain why the activity Design has zero float, it is easy to imagine what would happen if the activity were delayed by one hour. Instead of finishing on 9 February at 17:00, it would finish on 10 February at 09:00. Therefore, the successor activity, Foundations, would start at 09:00 instead of 07:00, meaning the existing hour of float would have been consumed and the project would be delayed by one hour. As defined above, critical activities are those that, if delayed, delay project completion, and in this case a delay to Design would imply a project delay.
To solve this problem, Primavera allows the user to define the amount of float below which an activity is considered critical, or to define the critical path as the longest path. Primavera P6 also allows negative numbers to be used as the limit for considering an activity critical. This is very useful when certain constraints create negative float, providing strong control over the critical path.
Primavera P6 also allows subcritical paths to be visualised using the Multiple Float Path option multiple float paths in p6. The more subcritical paths there are, and the longer they are, the greater the risk of delay to the project.
MS Project
MS Project, unlike Primavera P6, is much more intuitive, easier to learn and better for project reporting. However, because Primavera is database-based rather than file-based, it can manage projects with thousands of activities more efficiently.
For small projects or level 3 schedules in tender phase, MS Project is the most common software. Its similarity to Excel allows people who are not specialised in project planning to create and manage a schedule.
MS Project calculates the float of each activity in a similar way to Primavera and identifies activities with zero float as critical. As in Primavera, it is possible to modify the float value used to define critical activities.
As CPM software, Project has several important limitations:
- It does not allow the use of negative float.
- It does not allow longest-path calculation.
- It does not calculate multiple critical paths.
Although it is possible to activate Calculate multiple critical paths multiple critical paths in microsoft project under File > Options > Advanced, all this does is mark as critical any sequence of activities that finishes without a successor.