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Project Scheduling Negative Float

Recently, discussions have arisen regarding negative float in project scheduling, including its advantages and disadvantages. In this article, we aim to examine this topic and determine whether having negative float in a schedule is beneficial or detrimental.

What Is Negative Float, and When Does It Occur in Project Scheduling?

To begin, let’s review the concept of float. Float, also known as Total Slack or Total Float, represents the amount of time an activity can be delayed without impacting the project’s overall completion time. When the total float for an activity is zero, that activity is referred to as a “critical task.”

The calculation for total float can be expressed using the following formula: Total Float = Late Start – Early Start = Late Finish – Early Finish

Negative float occurs when the latest possible start or finish date of an activity falls before the earliest possible start or finish date.

The question is, under what circumstances does this situation occur?

It’s important to note that negative float is never a result of the type or number of relationships defined between activities. Negative float occurs when scheduling constraints or limitations (Constraints) are used.

Specific conditions are met:

1. In constraints such as “Start On,” “Start On or Before,” and “Mandatory Start,” when the constraint date is earlier than the Early Start (ES) date.

For instance, according to the prerequisite relationships, the Early Start date for an activity is the 10th day of the month, and you specify a start date as the 5th day of the month using one of the mentioned constraints. In this case, you introduce 5 days of negative float for the activity.

2. In constraints such as “Finish On,” “Finish On or Before,” and “Mandatory Finish,” when the constraint date is earlier than the Early Finish (EF) date.

Project Scheduling Sometimes:

External relationships (External Link) can also lead to negative float. This means that a prerequisite relationship between an activity in one project and another project can result in negative float. Negative float of -5 means that if this activity is completed according to its estimated duration, the project will be delayed by 5 days. In other words, to complete the project as scheduled, this activity needs to finish 5 days ahead of the planned schedule.

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In summary

Negative float can be a complex issue to manage in project scheduling, often resulting from the use of constraints or external relationships. It can disrupt the logical sequence of the scheduling network and lead to undesired effects when updating the schedule.


1. Minimize the use of constraints, especially for anything beyond specific milestones.
2. Instead of using negative float to highlight delays, consider tracking the difference between the Baseline Finish and Early Finish dates and implement catch-up plans if needed.

It’s essential to approach project scheduling with care and avoid the overuse of constraints that can lead to negative float, which can complicate project management and execution.
Please note that the U.S. Department of Defense (DOD) has prohibited the presence of negative float in scheduling as one of the recognized standards in the field of project planning.

The Negative Impact of Constraints on Scheduling It’s crucial to address one of the common mistakes made by non-professional planners, which is excessive use of constraints in project scheduling. Instead of establishing proper and logical relationships between activities, they rely on constraints to fix activities to specific dates.

It is recommended to avoid this approach and, instead, create an accurate schedule by defining precise predecessor and successor relationships, as well as activity durations. When planned dates for activities are not contractually mandated, a thorough review of relationships and consideration of logical overlaps or adjustments to estimated durations can help ensure that the schedule aligns
with the intended dates.

Constraints should generally be used sparingly, primarily for specific milestones. Rather than highlighting delays using negative float, it is advisable to track the variance between the Baseline Finish and Early Finish dates and develop a catch-up plan if needed.

Project Scheduling Negative Float :

Negative float can disrupt the logical network of scheduling and result in undesirable consequences during schedule updates. To illustrate this point, let’s consider an example: Imagine the construction of a stadium for the FIFA World Cup in Russia with a strong emphasis on completing activities before the start of the games. Even in the most challenging circumstances to meet the project’s deadline, instead of imposing strict constraints at the end of the project or phases, we can allow for delays and their impacts to be incorporated into the schedule. Following the PMBOK
guidelines, methods such as Fast Tracking or Crashing can be used to find solutions and develop a recovery plan.

Project Scheduling Negative Float In conclusion :

The U.S. Department of Defense (DOD) has deemed negative float in scheduling as prohibited, emphasizing the importance of sound scheduling practices in the field of project planning. Overuse of constraints and negative float can hinder effective project management and execution.

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Expected Finish Date in Primavera P6 is a type of mandatory finish constraint. It differs from other finish constraints in that, when you reschedule the project schedule, the software calculates the Remaining Duration field. The main difference is that using Expected Finish Date doesn’t result in negative float for the activity.

Here’s an example to illustrate its application:

Let’s consider Activity A with an Expected Finish Date of 19-Jun-2018. When you reschedule the project to a date like 09-Jun-2018, the Remaining Duration field is calculated by Primavera as 8 days.
Now, let’s say you have another activity, Activity B, which hasn’t started yet, and you set an Expected Finish Date for it. In this case, that date is considered a fixed and immutable date. It won’t change even if the predecessor activity (Activity A) is delayed. Therefore, the duration of Activity B will reduce proportionally to the delay in the preceding activity (Activity A).

Essentially, the key date, the Expected Finish Date, is fixed and unchangeable, locking the activity’s finish to that date. It serves as a date by which the activity must be completed.
Another use for Expected Finish Dates is with Long Lead Item (LLI) activities, which involve long procurement processes lasting several months. In this case, instead of estimating the Remaining Duration after each schedule update, you can simplify your work by considering this Expected Finish Date.

Overall, Expected Finish Dates can be useful when you need to set strict, unmovable deadlines for specific activities or when dealing with long procurement processes where the exact duration may be challenging to estimate.

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Author: Arash Beyazian Serkandi & Hamid Hoseinnasab

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Topic: Negative Float, Good or Bad?