Do your schedule activities have high float? High float is generally thought to be a good thing. It’s like a safety margin. But it may be indicative of missing relationships.
The Defense Contract Management Agency (DCMA) developed a definitive 14-point assessment for gauging the quality of a schedule. The intent of this assessment is to confirm schedule success prior to implementation or to provide schedule recommendations for improving the opportunity of project success.
The high float assessment counts the number of incomplete tasks that it considers high float, which it defines as tasks having total float greater than 44-days. If more than 5% of incomplete tasks are high float the schedule is flagged for further review.
This article discusses the DCMA high float assessment as one measure of schedule quality.
Okay, DCMA says the maximum acceptable task total float is 44-days. Well, why one single value? Why doesn’t the maximum acceptable task total float duration adjust or scale with the size of the project? It appears to be one size fits or doesn’t fit all schedules.
Well, the illustration I like to use is a forest. Let’s say you are in a helicopter flying above the forest. Regardless of the size of the forest, vast or small, you have to be at about the same height or altitude above to distinguish the trees from the forest. In schedule analysis, regardless of the total duration of your project, you want to be at the appropriate height or scale for viewing and understanding tasks and their associated predecessors and successors.
One may still ask: why is 44-days total float the derived cutoff point for achieving this perspective? Well, there appears to be nothing magic about the length of 44-days except that is comes to about two months, or roughly two reporting periods in a typical earned value management integrated system. Two months total float duration is a value, most likely, derived from practical experience and/or analysis on schedules of varying length.
We are taking DCMA’s 44-day high float definition on face value. Again, a maximum 44-days total float appears to provide about the right zoom or panorama for analyzing the schedule. Thus, the restrictions on activities having greater than 44-days total float.
But shouldn’t positive total float always be considered good? The total float of a task is the number of days that it can be delayed without impacting successive activity dates. Yes, project managers prefer tasks that have positive total float as it means that task may delay and still not negatively impact the end date of the schedule. Positive total float is like a built in safety margin. If a task does not progress according to plan it still might be acceptable on the time scale, as per the remaining value of its total float. So positive total float is good
But why are high float tasks unfavorable or suspect? A high float task may indicate missing predecessors and/or successors, and that the schedule network may not be logic-driven. In light of these high float possible negatives we recommend schedulers filter all tasks according to total float, and flag all high float tasks for review. Inspect the task logic to ensure that predecessor and successor relationships are properly defined.
An example of a high float task missing a successor is an install perimeter fence task on a construction site. On this project the install fence activity has, let’s say 60-days total float. This means the install fence activity can delay 60-days and still not negatively impact the conclusion of the project.
In our example, note that the total float of install fence extends to the end of construction, which, realistically, it can. But this may not be our desire. Most likely, we want our contractor to install the fence earlier because it provides security to the job site. Thus, our dilemma: we have lots of total float, which is great! But we prefer less because we want to secure the jobsite sooner.
One solution is we could put a finish on or before constraint on install fence to get it built earlier. However, this may seem like an arbitrary date, particularly, if schedule progress does not go as planned. And it really does not accurately describe our schedule situation.
A better solution or a more descriptive picture of our intentions says install fence no later than permanent power is provided to the site. The thought is that the temporary power generator may be easier to secure at night than the actual building’s electrical room and complete power grid. So a perimeter fence is more critical for the construction site once permanent power is installed and activated.
Let’s investigate install perimeter fence in light of this more accurate narrative. Analysis of our high float fence construction task reveals it is missing its link to the true successor activate permanent power. Completion of install fence should precede activate permanent power. Thus, we have a situation where a task has high float because it really is missing a successor. And it is not properly depicting the true portrait of the schedule. In this case the schedule is not driven by the logic of securing the construction site perimeter on or before permanent power is provided.
Provided that a critical path has been established, positive total float is good. It’s what project managers are hoping for. It provides a “cushion” for unpredictable delays. Particularly high float tasks, however, can be prognostic of missing predecessors and/or successors. The DCMA 14-point assessment proposes that tasks that have total float greater than 44-days (2-months) are suspect, and should be examined closely. The DCMA 14-point assessment does not forbid high float activities; it limits their use to no more than 5% of incomplete tasks. They are limited even in logic-driven schedules, perhaps, for no other reason than that they make it more difficult to track and/or understand the true story of the schedule.
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