Common Barriers to Valid Float
Unreasonable amounts of total float usually result from missing or incomplete logic rather than acceptable periods of potential delay. Therefore, any activities that appear to have a great amount of float should be examined for missing or incomplete logic. Because total float is calculated from activities’ early and late dates, it is directly related to the logical sequencing of events—just like the validity of the resulting critical path. Missing activities, missing or convoluted logic, and date constraints prevent the valid calculation of total float, potentially making an activity appear as though it can slip when it actually cannot. The reasonableness of total float depends on capturing all activities (Best Practice 1) and properly sequencing activities (Best Practice 2). It also depends on realistic resource assignments (Best Practice 3) and accurate status updates (Best Practice 9). Case study 13 shows how unreasonable float values can arise from improperly sequenced activities.
GAO’s analysis of FAA’s Collaborative Air Traffic Management Technologies system IMS found unreasonable total float throughout the schedule. For example, 325 (68 percent) of the 481 remaining activities had float values greater than 1,000 days. These unreasonable float values were caused mostly by activities tied to the project’s finish milestone, which was constrained to start no earlier than July 1, 2016. Interim milestones that were scheduled to finish earlier than July 1, 2016—such as those marking the end of task orders—were tied to the project’s finish milestone as predecessors and were therefore showing enormous amounts of float that did not reflect actual flexibility in the schedule.
The majority of high-float activities were level-of-effort activities, many of which were extended to the end of these interim milestones and thus associated with unreasonable float as well. Several activities had more than 1,000 days of float, including “test and deploy,” which showed a total float value of 1,280 days. This excessive float meant that delays in the activities would have no effect on the finish date of the Release 5 end milestone.Scheduling software automatically calculates total and free float for activities, which are then used to identify critical activities. However, these values of float must be examined for reasonableness. Unreasonable float might be negative, positive, or zero. That is, a network that displays large negative values of total float, such as -300 days for some activities, most likely indicates either missing logic or an unrealistic sequencing of activities. For example, dangling logic can create unrealistic free float. Because float is shared along activity paths, finding and addressing incomplete logic that causes large float values may solve the float problem for many activities on the path. Likewise, a complex schedule whose majority of remaining activities is critical is not a realistic plan and should be assessed for reasonable logic, the practicality of resource assignments, or the reasonableness of the project’s duration.