Forward and backward pass in CPM / PERT. (Apr 2003) (Oct 2006)
We use forward and backward pas in CPM / PERT to find Earliest and Latest occurrence times of events.
Forward pass calculations are used to find Earliest occurrence times of events (Ei). Forward pass calculations are from Left to Right. Start value is equal to zero. (Earliest occurrence time of 1st event = 0).
When two or more activities merge in an event, the maximum value is taken as the Earliest occurrence time for that event.
Forward pass time= Earliest time of Tail event + Activity time
Backward pass calculations are from Right to Left. Backward pass calculations are used to find Latest occurrence times of events (Li). For the last event, Latest time = Earliest time. If there is more than one activity coming back in an event, in backward pass we take minimum value.
Backward pass time= Latest time of Head event – Activity time