Two + two types of Float
When using the Critical Path Method (CPM) in traditional project management software, Total Float (TF) and Free Float (FF) are calculated, which most project managers know and use. But there is more to it. We can gain deeper insight in which timeframe an activity can shift, and the effects on the related activities by calling in two more types of float. The Interfering Float (INTF) and the Independent Float (INDF). Let me tell you how to interpret them, help decision making, and support claim prevention/preparation by using these four characteristics.
Intermezzo: Critical Path Method (CPM) Scheduling approach
As a short intermezzo, I’d like to address the Critical Path Method (CPM) for those who don’t know it. CPM is an algorithm for scheduling project activities. It had been developed for calculating schedules by hand before computer technology came to the play. It basically calculates the earliest dates activities can start/finish, taking all activities’ durations and dependencies into account. The calculations start from the project start milestone, on day one. This is called the forward pass. Secondly, we reverse the calculations. From the resulting earliest project finish date, it calculates the latest dates activities can start/finish to maintain this project finish date. This second step is called the backward pass. The activities where forward and backward pass have the same start and finish dates, embody the critical path.
Now, let us understand the purpose of the four types of float by looking at their definition:
Total Float (TF):
The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.
Mathematically: Late Finish – Early Finish = Total Float
Free Float (FF):
The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint.
Mathematically: Earliest Successors’ Early Start – Activity’s Early Finish = Free Float
Interfering Float (INTF):
The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date, but delaying an activity into interfering float will delay the start of one or more following non-critical activities. If an activity is delayed for the amount of the Free and Interfering Float, its successor activities are critical.
Mathematically: Total Float – Free Float = Interfering Float
Independent Float (INDF):
The maximum amount of time an activity can be delayed without delaying the early start of the succeeding activities and without being affected by the allowable delay of any predecessor activity.
Mathematically: Earliest Successors’ Early Start – Earliest Predecessors’ Late Finish – Activity’s duration = Independent Float
remark: when the result is a negative value, we set the value to zero.
The figure below schematically explains the difference between the four types of Float.
Figure 1: Schematic representation of TF, FF, INTF, and INDF.
Analysis and ownership
The Independent Float is the most restricted float and thus the (only) one genuinely owned by the activity and its owner. The consumption of the Independent Float does not affect the surrounding activities’ dates in any possible way.
You’d say the Free Float has the same characteristics on this matter. But the Free Float can be consumed by its predecessor activities, if they delay. You could say it’s owned by the predecessor activities as much as it’s owned by the activity itself. The float consumable by predecessor activities is schematically referred to as the Free Float Late in Figure 1, full consumption makes the observed activity critical.
If by delaying, the activity has consumed its Free Float (Early) and delays even further, it’s consuming the Interfering Float. The activities further up in the schedule logic will not be able to start on their early start date, but will not become critical until all interfering float has been consumed. This could lead to complex discussions concerning responsibilities in claim analysis.
The Total Float is simply the sum of the Free and Interfering Float. Mostly the Total Float is owned by the project and can be consumed either by owner or contractor on a ‘first-come-first-serve’ basis. Because of this fact, contractors often hide the Total Float by extending their activities in the schedule, manipulating the critical path. Any delay on the critical path, delayed by the owner or external factors, may give entitlement to Extension of Time (EoT) and can add strategic benefits.
The proper analysis of these parameters is totally dependent on the quality of the schedule. For instance, if the schedule does not consist of a closed network and has open endings, the float calculations have no added value. We refer to the blog post on the DCMA 14-point schedule assessment for a deeper understanding of how to build a good schedule.
These parameters, some calculated by your scheduling tool of preference, are both indispensable for steering and thus acquiring project success. This conclusion is only valid if your schedule is of good quality. For the float calculations, keep in mind to create a closed network schedule, without open ends and optional hard constraints on critical milestones. Building a good schedule can have great strategic advantages when EoT-claim analysis is performed. It’s important to have insight in these parameters when filing your baseline schedule or analyzing you contractor’s baseline schedule.