How do you display multiple float paths in P6 to provide a more complete view of the probable pathways through network logic when there are several paths close in duration? Here we take a look at how to do this.
The critical path is an important concept in scheduling a project. By most standard critical activity definitions, the critical path is the path through the network where a delay of any activity along that path delays the end date of the project. Project managers want to watch the progress of critical path activities; they drive the project end date.
In tight multiple-path schedules, however, they may want a more in-depth analysis of the differing paths through the schedule. It may be helpful to know the second or third longest path, which may become the critical path due to activity delays.
This article discusses how to generate a report displaying more than one pathway through the network logic to provide a more complete picture of likely schedule outcomes.
Multiple Float Paths
P6 Professional has a ‘calculate multiple float paths’ setting in the advanced schedule options that can generate a report displaying numerous float paths through the schedule. This Multiple Float Path (MFP) analysis differs depending on whether the last task of the float path is the end-of-project or an interim task. The analysis also depends on the selection of Total Float or Free Float options. Let’s demonstrate this.
Demonstration: Calculate MFP – Total Float
We first want to show the MFPs analysis when the last task in the float path is the end-of-project task, and the calculation is for Total Float.
In Figure 1, we have our demonstration project.
The task-dependent activities in this schedule are all linked with Finish to Start (FS) relationships. Note that task H has three predecessors: B, F, and G. The schedule has the below four paths from project start (PS) to project complete (PC):
The critical path is through PS-A-B-C-D-E-PC. We want to investigate other paths that are not critical but require attention. These are referred to as subcritical paths.
We proceed and calculate MFPs for this schedule.
- Click Tools | Schedule.
- Click the Options button, then select the Advanced tab.
- Toggle on ‘calculate multiple float paths’, Figure 2.
- Again, Figure 2, in the ‘calculate these paths using’ toggle on Total Float.
- Choose the ‘Project Complete’ finish milestone for the end-activity, Figure 2.
- Finally, in Figure 2, increase the number of paths to 4.
- Close the dialog and save the changes.
- Calculate the schedule.
This populates the Float Path and Float Path Order columns in the activity table, Figure 3.
This is a straight list of tasks. We want to display banners to help better organize the paths. This is possible using the Group & and Sort tool.
Demonstration: Group and Sort MFPs
Let’s proceed and use the Group and Sort feature to organize and display the float paths with banners.
- Choose View | Group and Sort or the Group and Sort button in the Layout Tool Group, Figure 4.
- In Figure 5, click the Sort button.
- Delete the sort by Activity ID, Figure 6.
- Add to sort by Float Path Order, Figure 7.
- Click OK.
- In Figure 8, change the Group By to Float Path, toggle to display Title and Name/Description, and click Apply.
P6 Professional groups the tasks for the first three Float Paths in the schedule, Figure 9.
P6 Professional only displays each task once in the output. Note that task A is a part of three float paths:
However, Task A in the above activity table and Gantt chart output, Figure 9, only appears in the float path 1 banner. From the output, we find that if a subcritical float path includes parts of the critical float path, only the tasks unique to that subcritical float path display in its group band. Again, this MFP analysis concluded at an end-of-project task.
What if we are interested in investigating float paths ending on tasks other than the end-of-project task? What if we want to review MFPs coming together on an interim task? We can do this by changing the task specified in the ‘display multiple float paths ending with activity’ setting to an interim task.
Demonstration: Driving Path to an Interim task – Total Float
Let us proceed and perform a Total Float MFP analysis where the float paths conclude at an interim task.
- Choose Tools | Schedule | Option | Advanced tab.
- Click the ellipse to modify the ‘display multiple float paths ending with activity’, i.e., the end-activity.
- Select task-I.
- Close the dialog and save the changes.
- Calculate the schedule.
The output is displayed in Figure 12.
Observe that the total float for each task in the PS-A-G-H-I float path is 0-0-2-2-2. Along the path, the total float increases from zero to two.
This indicates that the Total Float option in the MFPs analysis remains fixed on the project’s complete milestone, even though the end-activity in the analysis is not an end-of-project task; it is an interim task. Thus, we find that the Total Float setting in the MFP analysis is not meaningful when the float path’s end-activity is a project interim task.
What happens when we perform a Free Float MFPs analysis concluding at an interim task? Will the MFP analysis be accurate?
Demonstration: Driving Path to an Interim task – Free Float
We again perform an interim task MFP analysis, but we change the ‘calculate multiple float path using’ schedule setting to Free Float.
- Choose Tools | Schedule | Option | Advanced tab and toggle on Free Float.
We also remove the total float column from the activity table because it does not provide meaningful output for an MFP analysis when the float path’s end-activity task is an interim task.
- Calculate the schedule.
The final MFP analysis concluding at task-I is displayed as follows in Figure 14.
Observe the free float for each task in the PS-A-G-H-I float path is 0-0-0-0-2, Figure 14. The free float analysis provides more consistent results. The only issue is that the free float of task-I is 2-days, which is the outlier.
The free float is the amount a predecessor task can delay without delaying its successor task. But task-I should not have a successor. It appears that the analysis is saying that task-I has a successor, and it is the ‘Project Complete’ finish milestone. In the MFPs analysis to an interim task, this is not right.
It would make more sense if the free float of task-I remained unpopulated. However, the analysis leading up to task-I is still valid. Just ignore the free float value for the end-activity, task-I, because it is the end of the float path. It should not have a value.
Diverging Multiple Paths
The other issue is the float at the juncture where a path diverges into multiple paths. In this situation, a free float task along multiple float paths has multiple successors. What task is chosen as the float path successor for the free float in this situation? For example, task B in Figure 15 is on two float paths:
- PS-A-B-H-I and
Task B has successor tasks H and C: 1) BH has 1-day free float, and 2) BC has a zero-day free float. The successor generating the smaller free float is chosen, task-C. This task is on the float path PS-A-B-C-D-E-PC, which is listed as a ‘No Float Path’. Here ‘No Float Path’ means that the path does not have the end-activity task-I and is, therefore, not included in the Float Path and Float Path Order output.
Therefore, be warned that the designated free float value listed for a float path task may come from another float path. This float path’s end-activity may be an end-of-project task and not an interim task, as you requested in the MFP analysis. The P6 Professional free float output does not distinguish between end-activity successors on the interim task path and the end-of-project task path.
In our MFP analysis Free Float interim task output, the free float value of task B is not the delay between tasks B and H; it is the delay between tasks B and C, which is zero. And it is along the way of the ‘No Float Path’.
But in our MFP analysis, the interim task is task-I and the free float should come from the allowable delay between activity B and H, which is 1-day. So, the output is not accurate. However, note that the zero-day free float output between B and C, though less accurate, is the more conservative.
Demonstration: Adding a Relationship Changes Free Float Output
We want to investigate to confirm that the most conservative Free Float output is chosen where there are multiple float paths diverging at a respective task.
To begin, note the Free Float MFP analysis, the Free Float between tasks F and H is 3-days, which is correct.
Further, when we also link tasks F and C to create another connection to the ‘No Float Path’, the free float for F on ‘Float Path 3’ becomes 2-days. See Figure 17.
These 2-days are the total float between tasks F and C. This indicates that though not the most accurate, the most conservative free float value populates the free float field when a task is on multiple float paths.
Primavera P6 Professional provides in-depth float path analysis of multiple float paths. This way, the scheduler can examine both the critical and subcritical paths through the network. Perform MFP end-activity analyses to the end-of-project or an interim task. The Total Float MFP analysis only makes sense when the end-activity is the project finish milestone, an end-of-project task.
If you are examining an interim task for end-activity, then use the Free Float option and do not display the total float column in the activity table. Be aware that the Free Float MFP analysis has its own idiosyncrasies. If the free float to a successor task on the ‘No Float Path’ is the shortest duration, it is chosen, regardless.
So, when multiple float paths diverge from a task, the successor that has the smallest free float is chosen to represent the free float for the respective predecessor task. Again, when multiple float paths diverge from a single task, the MFP analysis uses the most conservative free float value.
You may want to consider reading Tom Blyles blog for more investigation on Primavera P6 MFP analyses.