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Negative float happens whenever you have some external time constraint that can't be satisfied by the plan. The simplest case is that you have an activity that's forecast to go on longer than the project's mandated end date.
Yes, it is possible to have negative float in the critical path. Keeping the "Golden Triangle" of Time, Scope, and Resources in mind, assume you have a project for a new widget to be added to an assembly that is due in 9 months. This due date is set in stone, if you deliver late there are catastrophic consequences.
At high level, here is your critical path
-Define the requirements of the widget (1 week)
-Design the widget and get approvals (2 weeks)
-3D print a prototype (1 week)
-Test the prototype, review, revise the design, retest (8 weeks)
-Procurement: Contract with a manufacturer that can produce the widget 2 weeks)
- Order Small Batch of Manufactured Prototype (2 weeks manufacturing, 1 week shipping)
-Test the manufactured Prototype (2 weeks)
- Secure certifications on the manufactured prototype (6 weeks)
-Order Large Batch of final widget (8 weeks)
-Manufacture final assembly with new widgets for customer order (2 weeks)
-Shipping lead time to customer (2 weeks)
Assuming each step cannot start before the previous step and no steps can overlap, this project will be one week late; you have one week of negative float. In order to bring the project in on time you'll need to review the steps to bring them in earlier, either by reducing the scope (fewer widgets? a design that is easier to manufacture?), adding resources (spend more money for expedited shipping or to get priority from the manufacturer?), or reducing the time for each step (fewer test cycles?, shorter design cycle? )
This is a very common problem for projects that involve the manufacture of custom equipment from third party vendors; the lead times are often extensive and depend on the availability of materials, so it is wise to work through the critical path regularly to check for negative float.
If it is a schedule you wish to baseline and commit to, this is a bad situation as a schedule that doesn't work on paper will not work in the real world.
If it is a revised forecast, then it means that you have a variance which you'd want to address assuming schedule performance is important.
This is a mildly provocative question; the answer depends on the answers to two others:
1. Are late (i.e. “finish no later than…”) constraints allowed to override the CPM late-date calculations? If so, then negative float can exist. (The dominant scheduling tools – like Primavera P6 and Microsoft Project – follow this rule. Some others do not.) Negative float indicates that at least one late constraint – on an activity or on the project as a whole – cannot be achieved as currently scheduled.
2. How is the “critical path” defined when negative float exists? Ignoring the effects of multiple constraints, multiple calendars, and other common complications, there are two primary alternatives that most professionals would recognize:
a. The critical path is the logic path – i.e. a particular sequence of activities and relationships – that determines the earliest completion date of the project. As long as the late constraint is applied to the end of the project, then this path would share the lowest total float value (i.e. the most-negative float.)
b. The “critical path” is not a logic path at all, but a collection of “critical” activities. These are typically defined – particularly in MS Project – as all the activities with TF less than or equal to 0.
Thus, if negative float is allowed to exist, then in the simplest case the critical path for the project is likely to include it.
For a more provocative question, substitute the word “positive” for “negative” in your original question, then be prepared for fireworks….
Linking some similar topics to help keep things connected.....
Yes. David and Scot made good points.
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