If you want to optimize your schedule it is advantageous to look to shorten activities that are along the critical path. But what do you do if you have a project constraint that causes your scheduling software to either display multiple critical paths or completely obscure the true critical path?
It is generally stated that an activity is critical if it cannot be delayed without impacting the schedule end date. Well, there are exceptions to every rule. And in this case the exception is that the activity may not be delayed without affecting an activity constraint date, such as a ‘Finish On or Before’ constraint date. Here the critical activity is not causing the project end date to slip, but an interim activity constraint date.
So a critical activity may not necessarily be on the longest path through the network. Refer to our blog for a detailed discussion on The Longest Path for additional discussions on how Primavera P6 handles these two distinct schedule calculation methods.
The Project Management Institute’s (PMI) “The guide to the Project Management Body of Knowledge (PMBOK Guide) Fifth Edition” defines the critical path as “the sequence of activities that represents the longest path through a project, which determines the shortest possible duration.” As the blog listed above indicates, critical activities do not necessarily have to fall along the critical path, i.e. longest path.
Note that the terms “critical path” and “longest path” are often used interchangeably. Technically however, they may not be the same. The critical path will show all activities with zero or less total float, which typically does contain a path through the entire network. However, the longest path is the most direct unbroken path from the first activity in the schedule to the very last, and as such may not display the same critical activities as a critical path
This is important to note because to optimize or shorten your schedule you want to shorten activities that are on the longest path through the network. Most schedulers want to display all critical activities whether they are on the longest path or not. So every activity that cannot be delayed without affecting either defined project interim dates or the project completion date will be flagged.
This is the default setting in most schedule software programs, and, in particular, Primavera P6 Professional. But if you hope to optimize your schedule then you want to focus in on the longest path alone
This article describes how to isolate the critical path in Primavera P6 Professional in support of schedule optimization efforts.
We begin with a demonstration research project schedule, Figure 1.
We have here two paths: one following experimental testing and the other following analytical modeling. These two paths converge at model analysis, where experimental and analytical data are compared. The normal total float default setting in P6 Professional defines critical activities as having zero or negative total float.
In light of this, note here that activities along the analytical path each have 3-days of total float and, therefore, are non-critical. Also, activities along the experimental path each have 0-days of total float and are critical. Most important, it is evident that the experimental path is the longest path through the network, and should be where we focus our optimization efforts.
Look what happens, however, when we place a project constraint on our schedule that falls before the natural network logic end date, Figure 2.
Suddenly, all our activities are critical and it appears we have two critical paths. So how do we know where to focus our schedule optimization?
Note we did not engage the Multiple Float Paths option in the advanced scheduling options dialog.
Let’s take another project constraint. Watch what happens when applying a project constraint that falls beyond the network logic, Figure 3.
This time all our activities have float, and the critical path is hidden. Again, were do we look to shorten the schedule?
In both situations we can find the longest path (or critical path) by switching the Schedule Options “Define critical activities as…” default setting to the Longest Path setting, Figure 4. To get to the Schedule Options dialog click the Schedule icon feature, and then select the Options button.
Too Many Paths
Now when it appears that we have too many critical paths we can find and display the true longest path, Figure 5, using the longest path setting.
Note in the figure that all activities along the longest path (shown in red) have total float of minus 3-days. Activities with zero total float, which are normally defined as critical activities by default, are not displayed in red because they are not along the longest path. Here only longest path activities are defined as critical, regardless of total float.
When all activities have float and the longest path (or critical path) is hidden we can, again, use the longest path setting to find and display the longest path, Figure 6.
In this example the total float of activities along the longest path is 1-day. Using the default total float setting these 1-day total float activities would not normally be defined as critical. But under the Longest Path setting they are critical, regardless of their total float. Other activities not associated with the longest path have total float of 4-days. Again, the defining factor is their path and not their total float.
What is the take away from this longest path input? Well, when you are in the process of optimizing your schedule you want to set the schedule options to longest path. This is particularly helpful for shortening schedules that have many activities.
Again, the longest path option defines all critical activities as being along the longest path. Later, when your schedule is optimized set the schedule options to define critical activities as having zero total float. This way you will flag all longest path activities and all activities up against an interim activity constraint date. So all activities in danger of delaying either interim constraint dates or the project end date are flagged.
Finally, make sure to document which schedule option’s critical activities definition (longest path or total float) are set for your schedule before sending it out to project stakeholders.