Forward Pass and Backward Pass to determine project duration
HTML-код
- Опубликовано: 11 сен 2024
- Forward Pass and Backward Pass to determine project duration
Forward pass is a technique to move forward through network diagram to determining project duration and finding the critical path or Free Float of the project.
Whereas backward pass represents moving backward to the end result to calculate late start or to find if there is any slack in the activity.
Let us try and understand few terms that as a project manager you will encounter.
How to plot Early Start (ES), Early Finish (EF), Late Start (LS) and Late Finish (LF) in a network diagram?
Early Start (ES) is plotted on the 1st left corner box at the top.
Likewise Early Finish (EF) is plotted on top right corner box.
Late Finish (LF) is on the right corner box at the bottom and Late
Start (LS) is plotted on the left bottom corner box.
Activity name “B” is in the 2nd box duration represented by 10 is on the 5th box at the middle.
What is Early Start?
Early Start (ES) represents the earliest start of an activity considering the dependency preceding task. If an activity is having more than one dependency predecessor, then ES will be the highest Early Finish (EF) of the dependency task.
Early Start = Maximum (or Highest) EF value from immediate Predecessor(s)
How to apply Forward Pass to calculate Early Finish (EF)?
In order to calculate Early Finish, we use forward pass. Means moving from Early Start towards right to come up with Early Finish of the project.
Early Finish (EF) = ES + Duration
If Early Start is 6 days and duration is 10 days, EF = 6 + 10 = 16 Days
What is Late Finish (LF)?
Late Start (LS) is the latest date that the activity can finish without causing a delay to the project completion date.
How to apply Backward Pass to calculate Late Start (LS)?
In order to calculate Late Start (LS), we apply backward Pass moving from Late Finish and deducting from activity duration.
LS = LF - Duration
If Late Finish is 30 days and duration is 10 days, LS = 30 - 10 = 20 Days
What is Critical Path?
Critical Path is the longest sequence of activity on a project that carry zero free float / slack.
Float Calculation
The whole idea of network diagram and finding the project duration is to identify the critical path and total float. Float represents how much each individual activity can be delayed without delaying successor activities or project completion date.
Total Float = LS - ES or LF - EF
Total Float shows the difference between the Earliest Start (ES) and Latest Start (LS) of an activity before the completion date is delayed.
Free Float = Lowest ES of successors - EF
Free Float represents the amount of time that an activity can be delayed before any successor’s activity will be delayed. A zero free float represents the activity is in critical path and there is no space to delay the activity without delaying the entire project.
Conclusion
Understanding forward pass and backward pass is crucial for a project manager. Although in a project management software, they are calculated automatically. But a project manager should understand how the system has calculated the values.
If you want to learn more on project management, please visit us at tiemchart.com/
Forward Pass | Backward Pass | Early Start (ES) | Early Finish (EF)| Late Start (LS) | Late Finish (LF) | Total Float | Free Float | Critical Path | Slack | How to calculate Free Float | What is Total Float
Tiemchart powered by Insiet ©
Are you project manager or business owner looking for a dashboard and reporting tool? Tiemchart is building customised reports and dashboard using the power of a BI Tools called Insiet © insiet.com/ . You may give it a try
Great explanation!
Made the concept crystal clear...
TQ sir. this tutorial is straight to the point
This gave me a better understanding of forward and backward pass. Thank you !
simple and clear, thanks!
Thanks it is quite clear 🎉
Thank you so much. This gave me the much needed understanding.
U deserve my like
minor correction at display time 6.28 on the free float = LS - ES instead mentioned FF= ES-LS
Thanks for the sharing!
isn't that for total float ?
Very simple
dont really get the rational of es/ef having same duration as ls/lf...is that something common?
kaka mai 500th subscriber hu
A question about the network paths:
Are these mutually exclusive, or should they be worked on in parallel? Expressed in project terms: Are *all* tasks of *all* paths needed to complete the project? /OR/ are the paths variants of each other, where only the best fit is chosen?
In this example: going the top path (A->B-D-G->H) will this be the whole project, and all that's needed?
That's quite abstract, hope someone can understand what I mean, thanks ... :D
Yes, all tasks in the network diagram need to be completed to complete the project.
In the example, Critical Path (CP) (A->B-D-G->H), will determine the shortest time (32 days) required to complete the project. In other words, you cannot complete the project in shorter time than this.
For any reason if you don't do any work on non critical tasks (C,E &F), then over the period, these tasks and the corresponding paths will become critical and extend the project duration beyond the original/baseline of 32 days.
For example: The Total float for task E is LS-ES= 19-9= 10 days. Therefore 10 days is the maximum time task E can be delayed. If it delays beyond 10 days, then this task will become critical extending the project completion beyond 32 days.
Please note that CP is dynamic in nature and will change during the execution of the project, as per the actual progress achieved. Therefore CP has to be monitored periodically (as per schedule tracking process) and mid-course correction(s) taken accordingly to keep the project on track.
Great presentation :)
Thanks, Big help!
B can start only after A is finish on 5th day, so B can start from 6th day only???
this was the issue I had as well. If you are using day 5 to complete task A - then task B cannot start until day 6. this is off by one day for forward pass according to my calculations.
I think so.
The difference between LS & ES or LF & EF is "TOTAL FLOAT" and NOT "FREE FLOAT", as explained in the video.
TOTAL FLOAT is the float/slack available for all the tasks combined in that particular path. Whereas FREE FLOAT is the float/slack available for an individual task in the path.
Learn to teach !!!! Rather than self teaching lol
That's a TOTAL float not FREE float :/