## Activity float

Whereas events have slack, activities posses float. The total float shown in Table 6.4 is the difference between the earliest start date of an activ ity and its latest start (or the difference between its earliest finish and its latest finish). It tells us by how long the activity's start or completion may be delayed without affecting the end date of the project.

 Activity Duration ( weeks) Earliest start date Latest Earliest Latest start date finish fate finish date Total float A 6 0 t mm 6 8 2 B 4 0 3 4 7 3 C 3 6 8 9 II 2 D 4 4 7 8 II 3 E 3 4 7 7 10 3 F 10 0 0 10 10 0 G 3 10 10 13 13 0 H 2 9 II II 13 t mm

Total float may only be Although the total float is show n for each activity, it really "belongs" to a path used once. through the network. Activities A and C each have 2 weeks total float. If. however.

activ ity A uses up its float (that is, it is not completed until week 8) then activity B will have zero float (it will have become critical). In such circumstances it may be misleading and detrimental to the project's success to publicize total float! There are a number of other measures of activity float, including the follow ing.

• Free float: the time by which an activity may be delayed without affecting any subsequent activity. It is calculated as the difference between the earliest completion date for the activity and the earliest start date of the succeeding activity. This might be considered a more satisfactory measure of float for publicizing to the staff involved in undertaking the activities.

• Interfering float: the difference between total float and free float. T his is quite commonly used, particularly in association with the free float. Once the free float has been used (or if it is zero), the interfering float tells us by how much the activity may be delayed without delaying the project end date - even though it w ill delay the start of subsequent activities.

Exercise 6.4

Calculate the free float and interfering float for each of the activities show n in the activity network (Table 6.4).