Phantom float in commercial scheduling software

Abstract

Abstract On a regular basis, construction professionals use scheduling software to resource load the schedules without paying attention to the resulting critical path. Current scheduling software fix the resource supply-demand problem by performing a Resource-Constrained Scheduling (RCS) technique, but 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 (phantom float). 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. This paper illustrates the presence of phantom float in Primavera's P6 and Microsoft's Project schedules. After removing phantom float from the schedule, non-critical activities may become resource critical and the actual float may be shorter than calculated or may be altogether non-existent. Users of P6 and MS Project should recognize that the calculation of total float by the software relies on a time-based context (LF – EF and/or LS – ES) ignoring the presence of resource constraints. Therefore, the float reported cannot be trusted or used to mitigate delaying events, like the traditional time-based context definition of total float suggests. Currently, research is being carried out in order to remove phantom float from P6 and Microsoft Project schedules.