Is your schedule critical path incomplete or, perhaps, disjointed? There may be a number of reasons your critical path is intermittent. The solution however, may be as simple as adjusting the critical task threshold.
There are legitimate reasons for gaps in your critical path. These include actual progress, which is displayed by blue bars. You may have a lag time between two critical activities, but that is okay. Also, you may have nonworking time, such as the weekend.
There are other issues that may be causing critical path gaps. One is you may have defined a constraint date on a task. Another is calendar discrepancies, where you have one activity assigned to a seven day workweek and another activity assigned to a five day work week.
The longest path scheduling option in Primavera P6 allows you to view the true longest path through the network to support schedule duration optimization. However, when you use the longest path option you may be unable see activities not on the longest path that are, none-the-less, critical.
A more complete solution is to adjust the critical task threshold, so you can see all critical activities regardless of whether or not they are on the longest path. You will, however, want to document this critical task threshold adjustment in your schedule memorandum and/or submittal.
This article explains how in Primavera P6 Professional to adjust the threshold for critical tasks to remove gaps in the critical path caused by either constraint dates or differing work week calendars. It is not a complete treatise on issues causing critical path gaps. Its intent is to demonstrate how adjusting the critical path threshold may provide for a complete critical path.
We have in Figure 1 our demonstration project schedule.
Note the green bars that are non-critical and red bars that are critical. Also, note the light blue center stripe on bar task ‘F – Cure Concrete’, which indicates that it has a unique 7-day workweek calendar. In addition to its light blue strip it also has green strips indicating that it is non-critical. You can see from the figure that there are gaps in the critical path. Critical activities are defined in this schedule as having 0-days total float.
The first gap comes after task ‘A – Prepare Agenda’; task A has 1-day total float. After Task ‘B – Meeting’ we have another gap. This gap starts at task ‘C – Set Forms’ and continues through task ‘F – Cure Concrete’. All these activities have 1-day total float, so they are listed as non-critical. The reason for the 1-day total float is the difference in assigned calendars.
Task ‘F – Cure Concrete’ has a 7-day per week cure schedule, which includes Saturday and Sunday. This makes sense as concrete cures 7-days per week. Task ‘G – Strike Forms’ has a 5-day per week schedule, which excludes Saturday and Sunday.
The 1-day total float comes in because task ‘F – Cure Concrete’ (7-day week schedule) ends on Saturday and task ‘H – Strike Forms’ (5-day week schedule) cannot commence until Monday. This results in 1-day total float between Task F and Task G. If task ‘F – Cure Concrete’ ended on Friday then task F would have 2-days total float.
Let’s change our criteria to define critical activities as being on the longest path. Select Schedule In the tools tool group, and click Options. In the Schedule Options dialog, Figure 2, toggle ‘Longest Path’ as the definition for critical activities.
Our schedule now is displayed in Figure 3.
Well, we’ve done much better this time. Tasks C through Task F are now shown in red, indicating they are critical. Now only Task ‘A – Prepare Agenda’ and Task ‘J – Safety Orientation’ are non-critical. Clearly ‘J – Safety Orientation’ is not on the longest path, and, therefore, is non-critical.
You would have thought that task ‘A – Prepare Agenda’ would be along the longest path. However, the Start On constraint placed on task ‘B – Meeting’ causes task ‘A – Prepare Agenda’ to fall off the longest path; it is non-critical by the longest path critical activity definition.
Let’s go back to our original technique of specifying critical activities by total float, Figure 4.
However, we adjust the critical task threshold to include all activities with 2-days (16-hours) total float or less. Why 2-days total float and not 1-day? Well, we chose 2-days or less as our critical activity criteria, so that if task ‘F – Cure Concrete’ ends at close of business, Friday, it will still be considered critical. If task ‘F – Cure Concrete’ ends on Friday it will have the entire weekend for float, which is 2-days.
When we raise the threshold for critical tasks to 2-days the resulting schedule is as displayed in Figure 5.
Now task ‘A – Prepare Agenda’, tasks ‘C – Set Forms’ through ‘F – Cure Concrete’, and task ‘J – Safety Orientation’ are all critical. And, we also have a complete critical path with no gaps.
The longest path option in the Primavera P6 schedule tool is helpful for isolating the true longest path for schedule optimization efforts. The longest path technique, however, has two drawbacks:
- Activities that have Start On constraints that are not along the longest path are displayed as non-critical.
- Predecessors of activities with constraints are displayed as non-critical.
When the longest path technique cannot fill in the gaps, Adjusting the Critical Task Threshold In Primavera P6 adjusting the critical task threshold can provide for a complete critical path. As mentioned previously, you will want to provide the value of the adjusted critical task threshold in your schedule documentation package.