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(PS-2752) Phantom Float in Commercial Scheduling Software

On a regular basis, professionals use commercial available software to resource load the schedules without paying attention to the resulting critical path. The studies found in the literature agree that Primavera™ is the most frequently used software in the construction industry, followed by Microsoft Project™.

These two programs fix the resource supply-demand problem by performing a Resource-Constrained Scheduling (RCS) technique. However, they report incorrect total float values and a broken critical path. RCS calculations suggest that activities have float but much of this float does not exist - hence, the name of phantom float. The aim of this paper is to illustrate the presence of phantom float in Primavera’s P6 and Microsoft’s Project schedules.

Phantom float is created in resource-constrained schedules because the existing RCS methodologies neglect the resource relationships that arise between activities when competing for the same but unavailable resources. Construction professionals as habitual users of scheduling software should recognize the presence of phantom float because it may lead them to make decisions based on unrealistic schedules. That is, non-critical activities may become resource critical and the actual float may be shorter than calculated or may be altogether nonexistent. Currently, research is being carried out in order to remove phantom float from P6 and Microsoft Project schedules.