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There are two terms related to Critical Path that one may encounter. These are the terms Forward Pass and Backward Pass. These terms are related to ways of determining the early or late start [forward pass] or early or late finish [backward pass] for an activity.
Forward pass is a technique to move forward through a diagram to calculate activity duration. Backward pass is its opposite.
Early Start (ES) and Early Finish (EF) use the forward pass technique.
To determine the Early Start of an activity, factor in all its dependencies and see its earliest start date.
Consider the following simple diagram (durations are in weeks):
Click to view original size
The Early Start (ES) for Activity B is 4. Why? B comes after A. A starts on week 1 and finishes on week 3. So the earliest that B can start is week 4. For simplicity, I think of it as: The duration of preceding activity + 1
The Early Finish (EF) is the earliest calculated time an activity can end. To calculate Early Finish, (ES for the activity + Activity Duration) - 1. From the diagram above, we can compute the EF of activity B as [(4 + 3) - 1] = 6. Hence, the EF for Activity B is 6.
Late Start (LS) and Late Finish(LF) use the backward pass technique. You can think of backward pass as calculating backward to see how much an activity may slide without affecting the finish date.
Late Start (LS) is the latest time an activity may begin without delaying the project duration. The simplest way one can compute the LS is adding the float to the activity Early Start. Using the simple diagram above, we know that Activity B is on the critical path, hence has a float of zero. Also, Activity B's ES = 4. Hence, LS = (0 + 4) or 4. Note that if an activity has a float of zero, ES and LS will be the same.
Late Finish (LF) latest time an activity may be completedwithout delaying the project duration. One can compute LF by LF =(Activity's LS + Activity Duration) - 1. So the LF of Activity B = (4 + 3) - 1 = 6. Note that since activity B has a zero float, EF = LF.
For memory trigger, if the float of the activity is zero, the two starts (ES and LS) and the two finish (EF and LF) are the same. Hence, If float of activity is zero, ES = LS and EF = LF.
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