PM – Forward and Backward Pass

While doing critical path calculation two Project Management terms (PM) will always come in picture. These are Forward Pass and Backward Pass. These terms are to figure out start and finishing time of an activity and its effect on critical path calculation.

Forward Pass : it is a mechanism to move forward through a diagram to calculate total activity duration. Backward path is other way around.

  • Forward pass is used by Early Start (ES) and Early Finish (EF)
  • Backward pass is used by Late start (ES) and Late finish (EF)

Let’s run through the following diagram to get more clear idea……..

Early Start (ES) activity is A as it is scheduled on third day where B is scheduled on sixth day.  For the second activity early start day is fourth therefore the activity C and E can be performed in parallel. The reason ES for second activity is day fourth is, activity A will take three days and at earliest C and E can be started in on fourth day. So the logic is “The duration of preceding activity + 1”

Early Finish (EF) is the earliest time that an activity can be finished. To calculate early finish we use (ES for the activity + Activity duration) -1. So early finish for activity C is day seventh. The reason is activity C will start on day fourth and will take total four day (fourth, fifth, sixth and seventh).  So the earliest time it can finish is on day Seventh.

Late Start (LS) is the latest time an activity needs to start without delaying the project. Late start can be calculated by adding float to the activity early start. As per the diagram above we know that activity E has one day float and activity E’s ES is 4. So LS for activity C will be 1+4=5. Similarly, we know that activity C is on critical path therefore has zero float and activity C’s ES is 4. So LS for activity C will be 0+4=4.

Late Finish (LF) is the latest time an activity can be completed without delaying the project. LF can be calculated by LF = (Activity’s LS + Activity Duration) – 1. So the LF of Activity E = (5 + 5) – 1 = 9.

Thats all for today…………….:)


About Mamun Shaheed

I love to imagine....I guess thats the only thing that keeps me going and going and going....
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