## 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 activity 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 Earliest Latest Earliest Latest Total (weeks) start date start date finish fate finish date float A 6 0 2 6 8 2 B 4 0 3 4 7 3 C 3 6 8 9 11 2 D 4 4 7 8 11 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 ll 11 13 2

Total float may only be Although the total float is shown 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, activity 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 following.

• 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. This 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 will delay the start of subsequent activities.

Exercise 6.4

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