Floats

In the simple network below the critical path is shown in red.

Any activity that is not a critical activity can be extended or delayed by a certain amount without delaying the completion of the project.  For example, activity C should start after 4 months and be finished after 6 months.  However, the finish could be delayed by 2 months, i.e. until the end of month 8, without affecting the completion time of the project as a whole.  We say that activity C has a total float of 2 weeks.

The total float of an activity is the amount of time by which it may be extended or delayed without delaying completion of the project, assuming no extension or delay in any other activity.  A critical activity can be defined as one which has a total float of zero.

There are two other types of float that are sometimes used.  To understand these, suppose the durations are changed in the example above.  The critical path is now A-C of duration 8 months, B now has a total float of 2 and D has a total float of 1.

The free float of an activity takes into consideration the effect on subsequent activities of a delay.  If B is delayed by 2 months it removes the spare time that was available for D, since if D was now also delayed by 1 the total project duration would extend to 9 months.  However, B can be delayed by 1 month without reducing the spare time for any subsequent activity. Therefore, B has a free float of 1 month.  D also has a free float of 1.

The independent float of an activity is calculated assuming the worst circumstances, i.e. the activity's predecessors finish at their latest times and we want subsequent activities to begin at their earliest times.  If this is possible, and there is still time to spare, then this is called independent float.  The free float of 1 month for B is also an independent float.  However, assuming the worst scenario in which B is delayed by 2 months then D cannot be delayed at all.  Hence D has an independent float of 0.

Copyright 2001 © Barrie Baker and  Neville Hunt, Coventry University
All rights reserved.  Last updated: 02 September 2005 .