How to avoid negative float in primavera time schedule

Negative float is an annoying problem in any, I will tell you how to avoid the negative float in a primavera schedule by applying these simple steps.

A- Reasons for negative float:

Negative floats mainly occur as a result of activity constraints. These constraints prevent the activities from moving forward as they are impacted by another activity or the project data date.

Let’s assume that the project activity is represented by a spring, this spring has a start point and end point (start date and finish date), if this spring was free to move from both directions and you pushed one end of it, it will simply move in the direction you pushed it on.

On the other hand, if this spring was fixed from one end (start date or finish date), and you pushed this spring, it will not move, and alternatively, it will be compressed.

This compression of the spring represents the negative float on an activity that has a constraint (i.e. must start by or must finish by).

B- How to avoid negative float:

We have to commit that date constraints are a fact on the construction projects, however, the method of applying these constraints may confuse the project schedule if we use the imposing tasks to must finish/start by no later than in primavera.

Alternatively, we can use a control milestone that links the activity required to start/finish on a certain date to another activity in the project as we will see in the example below.

C- Why we should avoid negative float in the primavera schedule?

Schedule negative floats give a misleading forecast date. For example, if you have a negative float in your schedule of -10 days, and the schedule is showing a forecast completion date of 1-Jan-2014, unfortunately, this forecast completion date is not correct and should be 1-Jan-2014 + 10 days.

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In complicated projects with a lot of activities with negative floats, it is hard to accurately define the project forecast completion date.

D- Example of avoiding negative float using control milestones:

Scenario 1:

Impose that the start date of the task investigating the groundwater levels to start on 12-nov-2013, then update the schedule with the data date 15-Nov-2013 which represents a 3-day delay to the task start date, you will notice the -3 days negative float.

Scenario 2:

 create an intermediate milestone linked to the project start date FS with a 6-day lag to control the start date of the task to investigate the groundwater levels on 12-nov-2013. Update the schedule with the data date 15-Nov-2013 which represents a 3-day delay to the task start date, you will notice that there is no negative float.

Note: Because my example schedule has only one activity, the forecast completion date has not been impacted, however, if the schedule contains many activities you will find a variance in the forecast completion date between scenario 1 and scenario 2.

This method should apply to all project management software such as Primavera, and Microsoft projects. Try this method by yourself and tell me your feedback.

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