A typical schedules critical path should consist of a continuous sequence of activities from project start or status date to the finish milestone. If not, your schedule may require review.
An unbroken and continuous critical path from project start to project finish is an important characteristic of a quality schedule. The critical path should have no breaks or unexplained large gaps of time. The critical path, however, may branch off into multiple critical paths, but these branches must all converge by project conclusion. So branch paths must also be unbroken. The ideal is a continuous logically linked schedule with finish to start (FS) relationships.
This article explains some of the more common causes of critical path breaks. The following is a list of common causes for critical path breaks:
- The activity start or finish is constrained.
- The predecessor activity has undocumented lag.
- An external predecessor drives activity start.
- Tasks are scheduled using different calendars.
- Resource leveling is causing delay gaps.
Activity Start Or Finish Is Constrained
In Figure 1 we have a pipe repair project created in Primavera P6 Professional.
This schedule demonstrates a continuous critical path from project start to project complete. However, watch what happens when we insert a Start On or After (SOOA) constraint on remove damaged piping, Figure 2.
We lose our unbroken critical path due to the constraint. This is not good. We need to investigate an alternative to this constraint.
Successor Activity Has Undocumented Lag
Lags on FS also cause apparent discontinuities in the schedule. In Figure 3 we have our pipe repair project and a seven day lag on remove damaged piping predecessor pipe inspection.
For some unknown and mysterious reason we delay remove damaged piping after the pipe inspection. A likely possible reason is that is the time required for order and delivery of a critical repair component. This may be the true situation, but nowhere on the Gantt chart or activity table is this indicated. We are left in the dark, as to the reason for the delay. And our lag between pipe inspection and remove damaged piping looks like a period of inactivity on the Gantt chart, which is why FS associated lag is listed among the causes of critical path breaks. You should seek to eliminate all FS lag in the schedule.
An External Predecessor Drives Activity Start
In Figure 4 we have removed the undefined lag and replaced it with an external delivery pipe & adapters constraint.
The start of pipe installation is driven by the external subcontractor effort delivery pipe & adapters, which are important parts for the pipe repair. Instead of the external predecessor delivery pipe & adapters driving the pipe installation we recommend defining the subcontractor order and delivery pipe & adapters effort directly in the schedule using a task dependent activity, Figure 5.
When we define an order and delivery task for the subcontractor effort we maintain an unbroken critical path.
Tasks Are Scheduled Using Different Calendars
If you really want to obfuscate your schedule assign multiple calendars to activities. Yes, you may have legitimate reasons for using more than one calendar, but avoid multiple calendars if at all possible. Assigning multiple calendars really breeds confusion.
One former client had a multi-hundred million dollar project on hold because of a longest path discontinuity. Their schedule was indicating a 44-day total float value along the longest path, which was a direct result of assigning multiple calendars to activities. The temporary solution was to insert a lag, but then their schedule review board wanted a stipulated valid reason for the lag insertion.
In Figure 6 we demonstrate how multiple calendars create schedule discontinuities.
In our demonstration most activities have a 5 day work week calendar. However, remove damaged piping has a 7 day work week schedule. The logic in our project schedule places the remove damaged piping activity on Saturday. Well, this means we have 1 day total float, on Sunday, between remove damaged piping and pipe installation. Our remove damaged piping 7-day work week calendar is creating a broken critical path situation in our schedule, which is a problem.
Resource Leveling Is Causing Delays
Leveling of assigned resources may shift activities and cause schedule delays and critical path discontinuities. In Figure 7 we have our schedule including assigned resources.
Note, in particular, the pipe fitter resource and associated activity assignments. The pipe fitter is scheduled to take vacation the third week of January, 2018. We assign the pipe fitter a resource calendar that shows no pipe fitter work the third week of January, 2018, Figure 8.
The resulting schedule is in Figure 9.
Primavera P6 Professional resource usage profile warns that the pipe fitter is over allocated. However, the schedule is not adjusted to remove the resource over allocation.
In Primavera P6 Professional when we make the activity type of these activities resource dependent the schedule automatically levels the resources. And shifts pipe installation and pipe & adapter installation activities to avoid the pipe fitter resource over allocation during the pipe fitter’s vacation, Figure 10.
Great! We have removed the resource over allocation. But now our schedule is vexed by another discontinuity. The takeaway is to inspect your resource assignments as a leveled resource may likely be the cause of the broken critical path.
Discontinuities on the longest path are cause for concern and may even result in suspension of work until resolved. Avoid use of internal constraints. External drivers and/or constraints are also a problem. Inclement weather calendars may be essential for your project, but multiple calendars dramatically increases schedule complexity and is therefore discouraged.
Leveling resources may also be the culprit causing the discontinuity. Discontinuity analysis is important and may make the difference between a schedule on hold and a schedule progressing according to plan. Knowing the list of broken critical path causes helps the scheduler in deciphering the mystery behind the discontinuity.