A Crash Course on Scheduling, Floats and Critical Path

Table of Content

A very common question amongst project controllers, construction dispute resolvers, and many other construction professionals is “Who owns the float?”. Unfortunately, this article will not be answering this question. Rather, it will attempt to explain what float is.

When someone mentions the “float”, oftentimes they are referring to the “Total Float”. In Project Management, there is another widely used type called the “Free Float”. This document will concentrate on these two main types of floats.

Total Float is defined as “the total amount of time that an activity within a schedule can be delayed from its early start date without delaying the project finish date, or intermediary milestone”.

Critical Path Method (CPM) schedules have an “early start date” and an “early finish date”. The early start date is the earliest possible date that an activity may begin, whilst the early finish date is the earliest possible time the activity may be completed – both of which are taken in relation to all of their preceding and succeeding activities. Conversely, late start dates are the latest an activity can begin without affecting the projects planned conclusion, whilst late finish dates are the latest an activity can finish without delaying the end of the project.

CPM software such as Primavera P6 or MS Project uses a complex calculation method called “forward and backward passes” to determine the early and late dates of activities within a set schedule, allowing planners, schedulers and programmers to forgo the need for manual calculation.

In order to fully understand floats however, we must also discuss schedules and critical paths. This is due to the fact that floats are derived from logically linked activities in a schedule. Schedules themselves consist of different paths, for which the longest path is the critical path. A simple illustration located below will be used to reference how a schedule is developed, how the “Critical Path” is determined, and how “Total Float” and “Free Float” are calculated.

The figure below represents a hypothetical train traveling left that is powered by two powerful rail engines. These engines pull a set of coaches on three separate tracks, which are represented on the horizontal grid and labelled as Track 1 (T1), Track 2 (T2), and Track 3 (T3). For each Track, there is an allocated space for 10 coaches, with these spaces represented by vertical grids labelled C1 to C10. Installed coaches that are to be transported along the tracks are represented by blue boxes with 4-digit alphanumeric codes which denote their position on this formed by the vertical and horizontal grid intersections, and are connected together by coupling (represented by green lines) that run from the front rail engine “SM1” to the back engine “FM1” alongst each track. Additionally, the yellow boxes represent missing coaches on the train. For example, coach T1C2 is a representation of Track 1, and Coach 2 whilst T1C1 is coachless.

To ensure that the train is sturdy while traveling along the tracks, there are interconnections between coaches on one track to coaches on other tracks. These are represented by the black lines shown on the train.

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A project schedule is broken down into work breakdown structures (WBS), which decompose the project’s scope of works into manageable sub-divisions which may be further broken down to other lower detailed levels to form a hierarchy of WBS’.

Tracks 1, Track 2, and Track 3 may represent different WBS’s of the same level in a schedule, with the coaches representing activities within them and each track representing a complete path. Each WBS and Activity usually has its own unique identifiers. It should be noted that all the coaches have unique codes which will represent unique activity IDs in a schedule.

Activities can either be classified as milestones, task dependant activities, resources dependent activities, or level-of-effort bars (for Primavera P6) and are connected in four different relationship types. These relationship types are;

  • Start-to-start
  • Finish-to-start
  • Finish-to-finish
  • Start-to-finish

The length of a coach may represent an activity’s duration, which may be expressed in hours, days, weeks, etc. Due to the aforementioned leftwards direction of travel, the green bar above coach T1C9 would be considered as being ‘first’, and would represent the start date of activity T1C9. The red bar on the right side of the activity represents the finish date of the same activity. This is the same for all activities.

The link between the start of predecessor activity T2C9 and the start of successor activity T3C10 is an example of a start-to-start relationship.

An example of a finish-to-start relationship would be the relationship between T2C7 (predecessor) and T3C10 (successor). As represented by the interconnecting black lines between the two, these activities on differing tracks are linked. In the illustration, activity T3C10 cannot commence until T2C7 is completed.

In another instance, the completion of activity T2C5 is linked to the completion of activity T3C6. This is an example of a finish-to-finish relationship. T3C6 cannot be considered complete until its predecessor, T2C5 is complete first.

The relationship between activities T1C9 and T3C10 are an example of a start-to-finish relationship. This relationship is such that, the start of predecessor activity T1C9 would trigger the completion of successor activity T3C10.

On track 3, the coach T3C5 is connected to the back engine FM1 through several activities. There are, however, four missing coaches between activity T3C5 and the front rail engine SM1. This chain represents a path in the schedule. There are other paths along the interconnecting logic and chains along individual tracks.

Free Float

Free-Float is defined as the amount of time an activity can be delayed without affecting the commencement of its successor activity. Applying this to the train illustration, when the brakes are suddenly applied, coach T1C7 will have to travel the empty spaces left in T1C5 and T1C6 before touching C4. This is an example of a free float. It can be said that there are two coaches (days/weeks/months) worth of free float between coach T1C4 and T1C7. It is important to note that free float is about durations between a predecessor and successor activities.

Total Float

If we only consider track 3 during a sudden application of the brakes of the trains, activities T3C5, T3C6, T3C7, T3C8, T3C9, and T3C10 will sit between start milestones SM1 (front engine) and finish milestone FM1 after travelling through the four empty coach spaces. The four vacant coach spaces represent 4 units of time of total float along the path of Track 3. The path T3C10, T2C7, T2C6, T2C5, T2C3, T2C2 and T2C1 has a total float of 3 days before impacting finish milestone FM1. This is due to T3C10 still being located after coach positions C8 and C9 (which accounts for two slots of time) in its connecting to T2C7.  There is also a missing coach (or time frame) at T2C4 along the path.

Critical Path

A schedule has critical and non-critical paths. A schedule may have two or more critical paths, which are paths which have no ‘empty coaches’. Considering the Path T1C10, T1C9, T1C8, T1C7, T2C6, T2C5, T1C4, T1C3, T2C2 and T2C1 (represented by the red line) between SM1 and FM1, one can notice that there are activities under each coach position along the path (10 coaches). As such, if the breaks are suddenly applied, there are no empty coach spaces for the path to move through, and hence no leeway. This critical path can be said of having a total float of zero.

Another critical can be identified along the paths T3C10, T3C9, T3C8, T3C7, T3C6, T2C5, T1C4, T1C3, and T2C2. This path T2C2 → T3C10 (may just be a few time units behind the critical path shown by the res line) considered to be a near-critical path in relations the aforementioned critical path. The critical path can change due to activities along the critical being completed earlier or delayed. When activities along other non-critical paths are delays, they may also take over the critical path.

Considering the two competing critical paths, if we assume activity T3C10 is delayed by another unit of time, then instead of the project finishing within 10 coach durations, it may now finish within 11 units of the durations specified with a total float of -1 if the finish milestone FM1 or back engine is constrained. Likewise, if activities T2C1 and T2C2 were both finished in one time unit instead of two time units allowed, the total float will be 1 day if now other activities takes over the critical path and FM1 is constrained.

With this brief article, you will be able to better understand Planners, Schedulers, Programmers and Delay Analysts whilst they explain their schedules.