I think the concept of float consumption is an interesting topic with a lot of variables to consider. This includes understanding the basics, such as the difference between free and total float, the different causes of consumption or changes over time, or what causes the float to grow. I know several schedulers who like to tout this metric as a key schedule risk measurement. I do fear that with all the variables at play that it can be misunderstood or misleading. To better improve this understanding and ensure that the float consumption metric is reliable, the schedule should first be mechanically sound (i.e., built using scheduling best practices).  Let’s start with the basics. What is the difference between free float and total float?

Free Float – The maximum amount of time by which an activity can be delayed beyond its early date without delaying any successor activity beyond its early date.

Total Float – The maximum amount of time by which an activity can be delayed without delaying project completion or another mandatory milestone.

Float is determined by completing a forward and backward calculation of the activities early start and late start dates, which are the basics for developing a CPM schedule. Many things can affect the calculation and the amount of float.  First, every planner/scheduler builds their schedules a bit differently, whether it is how they build their logic or how (i.e., use of relationships), if they resource load their schedules, and how they determine activity durations, etc.   Therefore, if three different planner/schedulers were given the task of building the same schedule you are likely to come up with three different float calculations. Similarly, they may have different critical paths and near critical paths.  Many planners may think to complete a forward pass in greater detail for near term activities and delay the detailing of downstream activities to a later date, especially activities like commissioning and startup for later in the project. Another scheduler may decide to expand the commissioning and startup activities, providing greater detail to ensure they have a clearer critical path. This could create a difference in activity end dates by days, weeks or even months, which could greatly affect the total float.  

Project schedulers may also use schedule reserve to create a buffer between the project finish date and the internal target date.  Some projects may add weeks or even months. This is similar to how we use contingency money to mitigate cost risks.   This could effectively hide some amount of total float depending on how the schedule reserve was deployed.  When it is deployed as a named reserve activity and therefore is clearly visible to the analysts, it is much easier to complete the analysis of float.  When it is buried into actual activities by adding a percentage or much worse, an irregular amount to activity durations, it becomes difficult to analyze the project’s true schedule performance as it was budgeted and determine the true critical path.  Analysis becomes even more difficult when a new planner scheduler takes over the planning/scheduling responsibilities from a departing one.

Since project schedules are most often managed through their critical and near critical paths, when looking at float density alone, it is common to observe project float compression.  Float commonly experiences some degradation through delays and changes in activity relationships, which is often added after a baseline is established to account for overlooked prerequisite tasks. Some tasks are not initially added as predecessors because they weren’t deemed likely to be driving activities when the baseline was established.  Therefore, it is also important to understand how both delays and schedule quality contribute to both float degradation and increased float density when doing schedule performance analysis.

On design projects, I always say that solid interdisciplinary handoffs are the key to success. Those should be the focal points because of ramifications of poor quality, poor communications or late handoffs. Knowing this along with the fact that the ability to start and complete an activity according to plan is also determined by the available resources (especially unique, critical, or scarce resources) and available information makes it a high-risk milestone.  Inefficient execution can often be attributed to the dearth of resources and information. 

Consider an interdisciplinary handoff with 0 days free float and 20 days total float, a delay of 5 days may not be alarming if you simply look at the float degradation.  However, the receiving discipline could experience an efficiency impact during the 5-day delay since the lack of the prerequisite design information reduces the number of work fronts available to them.  Additionally, the receiving discipline may also be impacted by moving 5 days of work to a later time and not having the right resources available to meet the new forecast, therefore reducing total float on the same or other path(s).  Considering this, it might be a prudent decision to delay another task with less float if it doesn’t present the same challenges to another discipline that isn’t as constrained with resource issues.

Construction projects face similar challenges as design projects.  Work front availability is determined by prerequisite activities.  Interfaces between crafts should be considered the same as inter-discipline interfaces in design projects because they may present the same resource challenges when experiencing quality or communications issues or delays.  Equipment delivery and setting planning presents an additional challenge as projects balance potential delays with having planned equipment available, versus substituting other equipment which may impact productivity and sometimes the critical path when it is determined by long lead equipment.

Float paths often benefit from segmentation into portions of similar resources.  A single discipline or craft can easily determine its own priorities.  It’s important to understand that a delay to an interface milestone is likely to create additional delays and collateral impacts.  Projects should consider both total float and free float relative to interface points of each float path to more effectively manage project priorities. This should just be one analytics tool in your “metrics” toolbox but it can be a helpful one if used properly.