When an activity has a negative Total Float value it means the activity cannot complete in time to satisfy an imposed Finish constraint on itself or on another successor activity along its path. Under this condition, the Late Start and Finish dates are actually EARLIER than the Early Start and Finish dates. In an expense loaded schedule, the effects of negative Total Float can be observed in the profile results on the Late Cost Distribution Curve.
In the illustration above, the Project has an imposed “Must Finish By” constraint date. Schedule Progress Update indicates that the Project will not finish on time evidenced by Activities with Negative Total Float values.
In this case, the resulting Late Cost values plots a curve that is ABOVE the Early Cost curve. In some instances, the Late Cost curve can even begin prior to the Data Date. The further left the Late Cost curve begins the more severe the Finish constraint has on the Cost profile and the more difficult recovery based on Cost will be. In addition, Late Cost Curves with Negative Total Float calculations have a tendency of flattening out prior to the Project End Date. Again, the earlier the Late Cost Curve flattens out, the more severe the Negative Total Float values exist on the Project.
In any case, the Early Cost curve remains the same regardless of Finish Constraints imposed on the Project. To produce an Expense Profile curve with a Late Cost Curve relative to the current status of the Project, remove the late constraints, the recalculate the schedule. This will remove the negative Total Float values and generate positive Total Float value.
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