What is Driving the Critical Path?
Do you ever look at a pdf of the schedule and see multiple activities in-progress that are also showing as critical?
How can this be?
Critical Path Defined
First, we need to know how the Critical Path is defined in the schedule (Primavera P6 Total Float and the Critical Path). If the Critical Path is defined as “Total Float <= 0 or a Total Float value”, and the project is complex enough to have more than one activity calendar, we really can’t be sure the Critical Path shown is critical to the constrained finish of the project completion or phase completion milestone. Or it just happens to have a Total Float value that makes it show as critical. This is especially true if the project is behind schedule. With this scenario, an activity may be critical or near critical or just report negative Total Float, but that doesn’t help us for Critical Path analysis.
I prefer to use “Longest Path” to define the Critical Path. This allows the Critical Path to be shown as the longest logical path through the schedule network to completion. This is the sequence of activities we can’t afford any slippage to.
Critical Path Driven by a Single Activity
Usually, there is one in-progress or un-started activity driving the Critical Path, so it’s not difficult to know which activities to focus our effort on to maintain or improve the scheduled finish date for the project.
Critical Path Driven by a Multiple Activities
But what about the Critical Path with more than one in-progress activity showing? Are they all driving?
Sometimes, there actually can be two activities which both drive the Critical Path. But this doesn’t happen that often. For the contractor who has the ability to view the schedule in native format, it is easy to see which activity(s) are driving the Critical Path.
I find that more often there are Start-to-Start (SS) relationships that allow in-progress activities, which do not drive any critical work, to show as critical. (The finish of these activities does not drive the start or finish of any critical activities).
How do we know which in-progress activity to focus on? If you only have a pdf to review, how do you know. (Many owners and owner rep’s do not have the ability to view the project in native format).
We can see that activity M-1010 shows as in-progress on the Critical Path. But is this activity driving the Critical Path? No, it is not. This activity has a SS relationship with successor activity M-1030, shown as being on the Critical. Both have started as sequenced, so the finish of activity M-1010 is not driving the Critical Path.
Activity M-1020 also has a SS relationship and 2 days lag, with the successor activity M-1030, shown as critical. The finish of activity M-1020 is not driving the Critical Path.
Activity M1000 has five successor activities. However, the critical successor relationships are the Finish-to-Start (FS) relationship and 2-day lag with activity M-1040, the SS relationship and 2-day lag with activity M-1020, and the Finish-to-Finish (FF) relationship with activity M-1010. (The finish of the activity with the SS relationship is not driving any critical work as both activities have started). Activity M1000 is the activity to focus on managing or pushing to maintain or improve the scheduled finish date. Slippage for this work could drive the start of M-1040 and the finish of M-1010.
Could you have determined this without the Successor information shown at the bottom of the Fig 8?
But How Do We Determine This With a pdf?
The truth is, we really can’t determine which activities are actually driving the Critical Path with a pdf. There could be a lag or FF relationship or both that actually do drive the finish of a concurrent in-progress successor relationship.
An in-progress activity could have a SS or FF relationship with a not-started successor activity making the use of dates to accurately “figure out” which activity drives the Critical Path on a pdf even less likely. Displaying the relationship lines on a schedule of any size is not a good option as the lines can overwrite each other limiting the value of this type of display.
We Need to Review Schedules in Native Format
We really need to review schedules in their native format. I prefer Primavera P6 but I also work with Microsoft Project. (Microsoft Project only allows the use of Total Float <= 0). There are other good programs available. Everyone has their favorite and each program handles the algorithm which determines the Critical Path a little differently. Be sure to research before you purchase!
Trying to determine which in-progress activity is driving the Critical Path when the pdf has multiple in-progress activities shown as critical, can lead to a project team expending effort or cost on work, which does not return any gain on the scheduled finish date for the milestone or project.
Plan your schedule…
Paul Epperson CCM, PMP, PSP, PMI-SP
Paul has extensive experience as a Construction Manager. Over time, he became convinced that there is a critical shortage of skilled planning and scheduling professionals in our industry. In 2009, he backed away from his work as a Construction Manager and began focusing on planning and scheduling. He now serves our industry as a subject matter expert in this area.