Backward Pass

The process of calculating late finish dates and late start dates for the uncompleted portions of all schedule activities is examined by moving backwards through the schedule network logic from the project’s end date. Subtract the duration from the late finish date of the last activity and add one to get the late start date. The process of calculation is known as Backward Pass and is reverse of Forward Pass.

Under project management, a backward pass in the area of project management involves the calculation of late finish dates and late start dates for the portions of schedule activities that are not complete. We can determine late finish and late start dates by starting at the project‘s scheduled end date and working backwards through the schedule network logic. Usually, the end date is set by the assigning party, or it may be determined through use of a forward pass.

For performing critical path calculation Forward Pass and Backward Pass are the commonly used terms in Project Management. Forward Pass is the methodology to move forward through a diagram to calculate total activity duration whereas Backward path is other way around.

  1. Forward pass is used by Early Start (ES) and Early Finish (EF)
  2. Backward pass is used by Late start (ES) and Late finish (EF)
Illustration:
Early Start

In the given illustration, Early Start (ES) activity is A which is scheduled on the 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 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.

Early Finish

Early Finish (EF) is the earliest time that an activity can be finished.

Early finish  = (ES for the activity + Activity duration) -1.

Early Finish of activity = (4 +4) – 1 = 7 days

To find the early finish for activity C lets take activity C starts on day fourth and will take total four day.  So the earliest time to finish is on day 7th.

Late Start

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.

E has one day float so ES of activity E = 4 days.

So, LS for activity C  = 1+4 = 5 days.

Activity C is on critical path therefore has zero float, so ES of activity C = 4 days

LS for activity C  = 0+4 = 4 days

Late Finish

Late Finish (LF) is the latest time an activity can be completed without delaying the project.

LF of an activity is calculated as, LF = (Activity‘s LS + Activity Duration) – 1.

So the LF of Activity E = (5 + 5) – 1 = 9 days

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