Keynote: Safe Float—Float Adjusted For Uncertainty
June 05, 2023
Total float is a pivotal concept in scheduling invariably relied upon by schedulers even though total float is based solely on a deterministic schedule without any consideration of risk. To do away with this limitation, PMA professionals are developing a body of knowledge that we call float risk assessment (FRA). In FRA, schedule leeway is calculated with respect to a realistically likely completion date as float use is modeled in the schedule simulation. Safe float values are further calculated to consider all duration uncertainty and risk remaining in the schedule. In summary, safe float is total float adjusted for uncertainty.
Throughout the presentation, Dr. Gui and Dr. Puri cover four main concepts surrounding project completion predictability:
For additional information about this invaluable scheduling innovation, you can read PMA’s article titled Safe Start Date & Safe Float.
Dr. Ponce de Leon is recognized as one of our nation’s foremost planning and scheduling subject matter experts. His broad professional experience includes executive and senior roles as investor’s developer, program manager, construction manager, and EPC contractor planner/scheduler. He continually pioneers innovations in project management and has written widely on the use of CPM in construction contracts as well as on schedule, delay, and acceleration analysis.
Dr. Vivek Puri has significant experience planning and executing projects in both construction and information technology. Dr. Puri’s recent work involves research and development relating to NetPoint, NetRisk, and Schedule MD. Three innovative tools were developed in-house by PMA. His doctoral work was in the area of simulation applications for construction planning.
I think we are back to the drawing board in this area. In reality, if all parties to a contract (the contractor, the owner, and the subcontractors) want to have an honest assessment of how delay impacts your project, you will want to simulate your schedule and know your safe starts. Relying on critical path delays when there is little uncertainty on the critical path is not the best solution. The best solution is to look at how delay may impact your p-value and completion date. I think that it’s going to take us a while to figure this out and to have some cases where this is affirmed. At that point, there will be a whole new generation of schedule delay analysis.
First of all, if you want to use safe float, you’re going to have to risk assess your schedule. Total float is a 66-year-old, myopic view of a schedule. Safe float theory was introduced in a paper by Gong and Rowings in 1995; our use of it in schedule risk simulation is an innovation of its use providing more accurate results. So, I think that if we start to use total float month-by-month, occasionally we get safe floats, you can then realize you are impacting your p-value and breaching safe starts. You’re going to be required to accelerate your schedule to change logic because the chances are that you are already falling behind. Safe float is going to allow you to do this objectively versus just relying on one scenario.
Not really, since safe float is based on certainty and risk. Near critical activity is still based on one scenario. Safe float is based on every possible scenario on your schedule. This considers safe float being similar to total float, but taking into consideration the uncertainty of activities being near critical. This is based on safe float rather than just the float available/deterministic float.
Not necessarily. Consider cases where you might have certain risk impacts, which end up creating bimodal distributions for completion dates for projects. So, the simulation will take into account those kinds of risks and actually generate bimodal distributions on these situations.
This can be achieved by performing project scheduling using both safe float and total float. You’re going to have a case where you only do total float during the project. The results will come in after experiencing both safe float and total float.
I would say to revisit your safe float quarterly. If you have a major delay or risk, then you would need to restimulate your risk and refigure out your safe floats.