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. 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): Mathematically:
Late Finish – Early Finish = Total Float Free Float (FF): Mathematically: Earliest Successors’ Early Start – Activity’s Early Finish = Free Float Interfering Float (INTF): Mathematically: Total Float – Free Float = Interfering Float Independent Float (INDF): Mathematically: Earliest Successors’ Early Start – Earliest Predecessors’ Late Finish – Activity’s duration = Independent Float The figure below schematically explains the difference between the four types of Float. Fig. 1: Schematic representation of TF, FF, INTF and INDF. Analysis and ownershipThe 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. ConclusionThese 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. This blog is originally placed on Primaned.be on June 15. Total Float Is the amount of time that an activity can be delayed past its earliest start or earliest finish without delaying the project?Let us first look at the definition of all these three attributes. FloatFloat (also known as slack) is the amount of time by which the start of an activity can be delayed without delaying the project completion time. Every task will have following set of start and finish time.
Is the amount of time a schedule activity can be delayed without delaying the early start of any successor?Free Float, per definition, is the amount of time that the activity can be delayed before any successors will be delayed. Early Start/Finish times are used to calculate the Free Float values.
What is the amount of time that a task can be delayed?A float (or slack) in a critical path method (CPM) is the amount of time that a task can be delayed without causing any delay to Subsequent tasks and project completion date. 2.
What is the amount of time that a task can be delayed without affecting the subsequent task?In project management, float or slack is the amount of time that a task in a project network can be delayed without causing a delay to: subsequent tasks ("free float") project completion date ("total float").
|