Construction Scheduling. Why Work When We Can Wait?

When the baseline schedule is developed correctly, the schedule logic is complete, activity durations and activity calendars are assigned, resources with cost may have been assigned, and activity coding is developed and assigned.

All of this creates the schedule network. This schedule network is made up of many sequences (logic paths) of activities.

Once the schedule is calculated (forward and backward pass), the schedule network will provide scheduled dates for activities not yet complete. At any given time, a small percentage of activities will have scheduled dates which show they should be starting and completing within a defined period of time, (say this week or this month…).

Some of these activities will be on the Critical Path. Some will be on the Near Critical Path(s). Some will have Total Float values which allow them to remain unstarted or incomplete for a period of time dependent on the Total Float value.

We all understand that the work on the Critical Path needs to be driven at all times. We understand that work on the Near Critical Path(s) also needs to be driven so it does not lag and shift to the Critical Path.

What about the activities with Total Float values which allow the work for these activities to lag without affecting the Project Completion Date?

Should this work be executed when it is convenient? Should this work be pushed out until it is on the Near Critical path(s)?

Part of the CPM Schedule development process was the analysis of the resources for activities to verify a specific resource did not have more activities scheduled to be underway than the resource could support. This can be done manually or using software programs that run Monte Carlo analysis for resource leveling.

That said, why would we want to allow work to slip? It is my opinion that any work which can logically be executed should be executed as long as the resources required do not exceed the planned resources for that time period. But, sometimes it is better to add resources to execute the work for a variety of reasons.

By allowing work to slip, only because there is no schedule driven reason to execute the work at that time, the work will eventually push out until the activities with a specific resource assignment are stacked resulting in an overallocation condition which forces some of this work to push out due to lack of resources. Whether the schedule is resource loaded or not, we all know that a subcontractor can only do so much with the forces they have on the project at any given time.

Allowing the work to slip also increases the risk of a late completion due to the potential of shifting of the Critical and Near Critical Path(s).

Finally, allowing work to slip has a negative impact on the schedule cost curve and distribution plan. The project may be reporting an on-time completion and the Critical Path may have an acceptable Total Float value, but if work is pushing out, the completed values will produce an unacceptable Cost Variance (CV), Cost Performance Index (CPI), Schedule Variance (SV), and Schedule Performance Index (SPI).

Projects with poor Earned Value metrics seldom recover.

I say, if the work is available to be executed, the resources are available, and executing the work will not lessen the focus on the Critical and Near Critical Path(s), it is wise to work in where you can, when you can.

I’d love to hear what you think!

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Paul Epperson CCM, PMP, PSP, PMI-SP