Negative float is annoying problem in any time schedule, i will tell you how to avoid the negative float in a primavera time schedule by applying these simple steps.
A- Reasons of negative float:
Negative floats mainly occurs as a result of activities constrains. these constrains prevent the activities to move forward as they impacted from another activities or from the project data date. lets 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. In 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 which has a constrain (i.e. must start by or must finish by).
B- How to avoid negative float:
We have to commit that dates constrains are fact on the construction projects, however the method of applying these constrains may cause a confusion to the project schedule if we used the imposing tasks to must finish / start by and no later than in primavera. Alternatively, we can use a control milestone which link the activity required to start / finish on 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 time schedule?
Schedule negative floats gives a misleading forecast dates, For example if you have a negative float in your schedule -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. and in complicated projects with 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 start date of the task investigate the ground water levels to start on 12-nov-2013, then update the schedule with data date 15-Nov-2013 which represents a 3 days delay to the task start date, you will notice the -3 days negative float (slack).
Scenario 2 : create an intermediate milestone linked to the project start date FS with 6 days lag to control the start date of the task investigate the ground water levels on 12-nov-2013, then update the schedule with data date 15-Nov-2013 which represents a 3 days delay to the task start date, you will notice that 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 be applicable to all project management software such as primavera, Microsoft project. Try this method by yourself and tell me your feedback.