ES vs EF vs LS vs LF-Difference between activity ES,EF, LS,LF
This page compares activity ES vs EF vs LS vs LF and mentions difference between activity ES (Early Start), EF (Early Finish), LS (Late Start) and LF (Late Finish). The links to other difference between terms are also provided.
Early Start-ES
Early Start or ES is the earliest time that activity can start. An activity near the end of path or much later will only start if all the previous activities in the path also started early. If any one of the previous activities slips then it will push this activity out.
Early Finish-EF
Early Finish or EF is the earliest time that activity can finish. It is the date that an acitivity can finish if all previous activities started early and none of the activities slipped.
If project manager and team knows about early start and early finish dates of all the activities(tasks) then they will know how much freedom they have to move the start dates without causing any problem to the project.
Late Start-LS
Late Start or LS is the latest time an activity can start. If an activity is on the path which is much shorter compare to critical path, then it can start much late without any delay to the project. But these delay cause problem if other activities on the path slip.
Late Finish-LF
Late Finish or LF is the latest time that an activity can finish. If an activity is on a shorter path and all of the other activities on same path start as well as finish early, then it can finish very late without causing project to be late.
Knowing late start and late finish will help you see how much it can be played around in the schedule. An activity having large LS and LF means there are more options available.
Tips for calculation of ES,EF during forward pass
➨ ES of first activity = 1
➨ ES of all other activites = EF(of previous activity) + 1
➨ If two activities converge(i.e. merge) to one activity in forward pass as shown in fig-1,
then ES of activity C = (greater EF from activity A and B) + 1
➨ EF = ES + duration -1
Forward pass is the direction from left to right as you traverse on network diagram.
The example-1 below depicts ES(Early Start) and EF(Early Finish) calculations below.
Tips for calculation of LS,LF during reverse pass
➨ LF of last activity = EF of the same last activity
➨ For all other activities except the convergence mentioned in the next line,
LF = LS(of previous activity) -1
➨ If two activities converge(i.e. merge) to one activity in the reverse pass as shown in fig-2 then,
LF(of activity C) = (Lesser LS from activity A and B) -1
➨ LS = LF - duration +1
Reverse pass is the direction from right to left as you traverse on network diagram.
The example-2 below depicts ES(Early Start) and EF(Early Finish) calculations below.
Example-1 ES(Early Start) and EF(Early Finish)
If one need to calculate ES and EF from the above network diagram depicted in fig-3, follow tips
mentioned above.
➨ES of first activity is 1, EF =ES+duration-1 = 6
➨ES of activity-B = EF of activity-A +1 ,
Hence ES of activity-B = 6+1 = 7 ,
EF of activity B = 7+5-1 = 11
➨ES of first activity D = 1
➨EF of Activity D = 1+2-1 = 2
➨At activity-C two activities merge,
hence we need to take bigger EF from activity B(11) and D(2),
hence ES = Bigger EF +1 = 11 +1 =12
➨ EF of activity C = 12+7-1 = 18
Example-2 LS(Late Start) and LF(Late Finish)
➨ LF of activity C (last activity ) = EF of same activity =18
➨ LS of activity C = LF - duration +1 = 18-7+1 = 12
➨ LF of activity B and D = LS of C - 1 = 11 each
➨ LS of activity B = 11-5+1 =7
➨ LS of activity D = 11-2 +1 = 10
➨ Now LF of activity A is lesser LS from B or D
i.e. LF of activity A = lesser LS -1 =7-1 =6
➨ LS of activity A = LF-duration+1 = 6-6+1 = 1
Useful PMP Related Links
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