Critical Path

http://www.aacei.org/technical/rps/49R-06.pdf

Kandarp,

There are two ways of calculating total floats to determine critical path and longest path. It’s either Forward Pass or Backward Pass



Forward Pass

The purpose of the forward pass in CPM is to calculate the earliest start and earliest finish dates. The earliest start (ES) of a typical activity is the greater earliest finish (EF) of any of its preceding activities in the network diagram plus 1 time unit, or 1 if it is the first activity

For example, given a linear logic of activity A followed by activity B, ES(B) = EF(A) + 1 similarly, EF(B) = ES(B) + Duration(B) – 1 working from left to right, the logic of the network diagram is followed until all the earliest start and finish dates are complete.



Backward Pass

The purpose of the backward pass in CPM is to calculate the latest start and latest finish dates. The latest finish (LF) of a typical activity is the greater latest start (LS) of any of its succeeding activities in the network diagram minus one time unit, or the finish of the project if it is the last activity.

For example, given a linear logic of activity A followed by activity B, LF(A) = LS(B) – 1 similarly, LS(A) = LF(A) – Duration(A) + 1 working from right to left, the logic of the network diagram is followed until all the latest start and finish dates are complete.



Free Float & Total Float

Free float is defined as the amount of float that can be used up without affecting the earliest start of any other activity



Total float is the shared float for all activities along a particular network Branch.

Total float for any activity can be calculated by either one of the following equations (mathematically both are the same):

Total float = LS – ES

Total float = LF – EF

Activities with no (total) float are described as critical and lie on the critical path, and any delay to these activities will delay the project’s completion.

Kandarp,



Critical path is determined by total float which is equivalent to zero or more depends on how you define your critical path. It is not always the case that it is the longest path of the schedule.



It is not mandatory to attached finished milestone at the end to create critical path but it is recommended as basic and best scheduling practise to have at least start and finish milestones which is a part of level 1 schedule.



If finish milestone move after linking to the last activity therefore your finish date is also move due to logic. If you don’t want your finish milestone to move you can put a mandatory finished constrain but it is not recommended because it will distort the logic and will not give you the true critical path and you might end up of having negative floats on other activities. Constraining activities mostly used in analysis and or special scenario where you don’t have control on that task and it should be fixed.

For example a hand-over of another party which you don’t have control except for the fixed date given to you. Another is example is clients delivery of equipment giving you a fixed date which you don’t have control that prevent you from starting a work on that equipment.

Hi Kandarp



Basic planning principles state:



1. Every task must have at least one outgoing link to another task or to the completion date.



2. If your finish milestone moves to a later date because you have put in place the correct logic then that is what the planned completion date will be.



Best regards



Mike Testro