Primavera Professional P6 supports a couple of options when schedulers want to define a critical activity in the Gantt chart; Total Float or the Longest Path. So, what’s the difference between these two?
Total Float is the total amount of time that a schedule activity may be delayed from its early start date without delaying the project finish date, or violating a schedule constraint. If the Total Float value is zero then it is an indication that an activity is critical.
The Longest Path is the path through a project network from start to finish where the Total Duration is longer than any other path. The terms ‘Critical Path’ and ‘Longest Path’ are often used interchangeably by schedulers when discussing a project’s sequence of work to deliver on time. This can be confusing as there are differences.
To define Critical Activities, select the projects view and select a project. Click on the Settings tab as shown in the Figure 1 below.
Primavera P6 has a default set to the Total Float method, however you can change the setting to the Longest Path.
Which Method should You Choose?
Well, ultimately that is down to the preference of the scheduler and what they are trying to achieve. Below are two examples showing the different methods.
Total Float Method
In Firue 2, activity A1050 has a ‘Finish On or Before’ constraint that is equal to its planned finish date, therefore it and its predecessors are showing critical. If they slip, they probably won’t impact the end date of the project, but they will overrun their constraint date; which is why they are showing as critical from a ‘Total Float <= 0’ perspective.
Now let’s look at the identical example using the Longest Path method.
The Longest Path Method
The previously critical activities displayed in the Total Float method are now showing as non-critical. Even though they still have zero days Total Float, Primavera P6 ignores them as they are not part of the Longest Path through the network.
By using the Longest Path through the schedule, the network will only consider activities as critical if they are on a contagious path from the start of the project to the end. This is normally a single path which any activity that slips will impact the end date of the project.
When using Primavera P6 Professional, Oracle offers both ways to define a critical activity in a schedule. Schedulers often select the Longest Path method when they are developing a schedule, as this offers them a clear picture of the activities which are driving the scheduled finish date for the overall project.
They may switch to the Total Float method (this option is available at any time during the project life cycle) to view more complex constraint activities such as resource leveling, path divergence, relationships, and lag.