FLOAT :
Float or spare time can only be associated with activities which are non-critical. By definition, activities on the critical path cannot have float. There are four types of floats. Total Float, Free Float, Interfering Float and Independent Float.
(i) Total Float :
The total float of an activity represents the amount of time by which an activity can be delayed without delaying the project completion date. In order words, it refers to the amount of free time associated with an activity which can be used before, during or after the performance of this activity. Total float is the positive difference between the earliest finish time and the latest finish time or the positive difference between the earliest start time and the latest start time of an activity depending upon which way it is defined.
(ii) Free Float :
Free float is that portion of the total float within an activity which can be manipulated without affecting the float of subsequent activities. It is computed for an activity by subtracting the head event slack from its total float. The head event slack is the difference between the latest and earliest event timings of head event of an activity that is its (L-E)
(iii) Interfering Float :
Utilisation of the float of an activity may affect the float times of the other activities in the network. Interfering float is that part of the total which causes a reduction in the float of the successor activities. It is the difference between the latest finish time of the activity in question and the earliest start time of the following activity or zero, whichever is larger. It indicates that portion of the float of an activity which cannot be consumed without affecting adversely the float of the subsequent activities.
(iv) Independent Float :
This is the amount of time an activity can be delayed when all preceding activities are completed as late as possible and all succeeding activities started as early as possible. Independent float therefore does not affect float of either preceding or subsequent activities. It is computed by subtracting the tail event slack from the free float of the activity. If the result is negative it is taken as zero.
For examination purposes the most important type of float is total float because it is involved with the overall project duration. On occasions the term ‘Float’ is used without qualification. In such cases assume that Total Float is required.
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