Which one of the following statements is NOT correct related to total float?
- ATotal float of an activity is obtained by subtracting its early start time from its late start time.
- BSubtracting the early finish from late finish provides total float.
- CActivities with zero total float is called critical activities.
- DTotal float can be calculated by subtracting its early finish time from the earliest of the early start time of the activities directly following.Correct
Solution
Concept
Total float = LS − ES = LF − EF. The earliest ES of the following activities minus EF gives free float, not total float.
Formula
TF = LS − ES = LF − EF; FF = min(ES of successors) − EF
Step-by-step solution
1. Total float TF = LS − ES, so option A is correct. 2. TF = LF − EF, so option B is correct. 3. Activities with TF = 0 are critical activities, so option C is correct. 4. Earliest ES of the following activities − EF of the activity = free float, not total float. 5. So option D is NOT correct.
Explanation
Total float is the time an activity can be delayed without delaying the whole project. It is found as LS − ES or LF − EF. The formula in option D gives free float, which is delay possible without delaying the next activities.
Why the other options are wrong
A: correct definition of total float (LS − ES). B: correct, LF − EF also gives total float. C: correct, zero total float means a critical activity.
Shortcut
Using the successor's ES means free float. Using LS or LF of the same activity means total float.
Common mistake
Mixing up free float and total float.
Correct answer: D) Total float can be calculated by subtracting its early finish time from the earliest of the early start time of the activities directly following.