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Forward & Backward Pass

30 replies [Last post]
wilfred datinguinoo
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Hi everybody,

I’m still new in P3 can you help elaborate/explain
the significance of forward and backward pass and how to
use it?

Regards,

wilfred

Replies

Gary Whitehead
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A 10 year old thread skewing a tad off-topic, so I'm going to lock it.

Stephen Devaux
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I just have to ask: if we are talking about the forward and backward passes, why is the other item of info to be discussed total float? Should we not focus instead on the more important metric of critical path drag? More important because it's only ON critical path activities, while float is always OFF the critical path?

Additionally, because it delays project completion and time is money, drag almost always has a dollar value called drag cost. Time delay = value decay. Float does not, usually, have a cost (unless it gets used up and winds up becoming drag!).

Here is an article in Defense AT&L Magazine (PDF file) if you want to read more about it. And if you want even more about the use of critical path drag, as well as a lot of other new techniques and concepts, my newest book Managing Projects as Investments: Earned Value to Business Value came out two weeks ago from CRC Press. If you click on the Reviews tab, you will see that a lot of knowledgable people think it contains some valuable innovations.

Please don't misunderstand -- both total and free float are important scheduling concepts that the software should and does compute. But critical path drag is -- uh -- critical?

Fraternally in project management,

Steve the Bajan

Zoltan Palffy
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the forward pass begins at either the start date of a project (if there is no progress reported) or the data date of the projetc and goes forward in time taking into consideration the durations, calendars, logical ties and any hard constraint dates. This wil determine the EARLIEST that an activity can start and or finish. These are the early dates.

the backward pass begins at the project must finish by date (if no constraint date is used then it starts at the early finsh date) and goes backward in time taking into consideration the durations, calendars, logical ties and any hard constraint dates. This wil determine the LATEST that an activity can start and or finish. These are the late dates.

the difference betwen the late date and the early dates is the float.

 

Saqib Ali
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Hi Luca,

I read your coments today. So please can you send me your power point presentation. 

I'm interesting to study your presentation.

My mail id      saqib.ali221@gmail.com

 

Many Thanks in Advance

Muhammad Anjum
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Total Float = LF - ES - DURATION + 1

 

Muhammad Anjum

Muhammad Anjum
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Forward pass is used to calculate early dates and backward pass for calculating late dates of activities in the schedule sothat we can findout floats in the activities and critical path in the schedule.

Forward Pass calculates Early Dates:

EF (early finish) = ES (early start) + DUR (activity duration) - 1

Backward Pass calculates Late Dates:

LS = LF - DUR + 1

 

Regards,

Muhammad Anjum

Stephen Devaux
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Marwan, you are very welcome! Please let me know how using Drag works out on your contracts.  And please don't hesitate to contact me with any questions.

I truly believe that as planners start to use Drag and Drag Cost, three things will happen:

  1. Projects will become shorter.
  2. Projects will become more profitable.
  3. Planners will become more valuable and appreciated.

 

Fraternally in project management,

Steve the Bajan

Marwan Hmood
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Stephen. The DRAG of critical activities, is new to me. The article is very informative and will be agreat start to explore & probably use the drag concept in our current and future projects. Thank you.

 

Marwan

Charlie Collins
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Ronald,

Thanks for the link.  You provided the necessary explanation I had been craving.  I am one of those folks who did not know there was a difference in cpm calculation methodology. I was taught the Aggie method in school and used P3 in the construction industry, not fully understanding how the software calculated CPM. Just completed a Masters course in PM and the advanced scheduling class used the Aggie method along with Project 2007.  I believe the PMBOK may be smart in not favoring one method over the other because both seem to be used extensively.    I have recently left industry to teach construction management at a tech school and currently use the Aggie method.  We also expose the student to both P6 and MS Project after they develop their network working skills by hand.  Do you recommend any change in teaching methods (Aggie vs ACCE), or just simply show both methods and explain the difference between the two?

Again, I appreciate the insight!

Charlie

Stephen Devaux
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Marwan, one additional note: the formulae shown below for Forward & Backward Passes omit perhaps the most important calculation: Critical Path Drag.  Why the most important? Because whereas float is (usually) only on activities that are off the critical path, Drag is only on activities (or constraints) that are ON the critical path! 

Drag is the measure of the time an activity (or constraint) is ADDING to the project duration.  Not only is it essential knowledge for schedule compression (and/or recovery), but, because time is money, Drag almost always has a cost (sometimes a very large cost!).  The True Cost of any CP activity is the sum of its resource cost plus its Drag Cost.  Drag Cost can be extremely useful in justifying additional resources (to reduce True Cost).

Some articles on the subject:

"The Drag Effcient: The Missing Quantification on the Critical Path", Defense AT&L Magazine:

http://www.dau.mil/pubscats/ATL%20Docs/Jan_Feb_2012/Devaux.pdf

"Scheduling Is A Drag":

http://www.projectsatwork.com/content/articles/246653.cfm

"DRAG Racing on the Critical Path":

http://www.projectsatwork.com/content/articles/234282.cfm

The two ProjectsAtWork articles require registering on the site, but it's free.

Note: Primavera does NOT compute Critical Path Drag -- but that does not mean that it is not a critical metric! If the software does not compute Drag, then the planner needs to know how to compute it "manually".  Spider Project DOES compute Drag, and IMO that fact, in and of itself, is reason to use Spider rather than Primavera.  Also, there is an add-on to MS Project 2007 and 2010 that computes Drag: the Sumatra Project Optimizer from Sumatra.com.

Fraternally in project management,

Steve the Bajan

Marwan Hmood
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Hi,

Would also like to recieve this presentation to our email: abraj.consultants@gmail.com

 

Thank you,

 

Marwan

Ronald Winter
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Charlie, 

To answer that question, why not take a look at a paper that I published in the AACE International journal, Cost Engineering in 2003?  The paper is titled, “How to Befuddle a College Professor (without Really Trying)” and a copy can be found in my Published Paper Library at http://www.ronwinterconsulting.com/published.htm.  Good luck!

Charlie Collins
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Why does primavera use -1 in the forward pass and +1 in the backward pass?  The "by hand" calculations I have always been taught never used the -1 and +1 in the formula.  Please explain their reasoning.

 

Thanks,

Charlie

IC Quiamco
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Here are the basic of it.

Forward computation (ES + Duration - 1 = EF)
Backward computation (LF - Duration + 1 = LS)
Total Float= LF - EF

Forward Pass:
-Task A ES is always equal to 1
-Apply forward pass formula to get EF of first task.
-Task B ES is Task A EF+1 and so on.
-For a Task with multiple predessors with different EF, adopt the largest EF+1 and so on

Backward Pass:
-EF is always equal to LF at the end or last task of the project.
-Apply backward pass formula to get LS of the last task.
-The LF of the next task is just LS of the last task - 1.
-Continue the cycle until you reach the first task.

Total Float:
TF=LF-EF

Task with zero float "Critical".
Mortreza Shahrian
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hi luka

i aould like to see your presentation.
would you please to send me ?
my email is : morshah12000@yahoo.com

best regards
shahri
Forum Guest
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Hi, Luca.

Can you please send your presentations also to me. I was today on the way to find on the internet some data about it. And I have find this wonderfull webpage. It’s really great. I’m now Guest. I waiting only of my password and ID.

I will be very happy, if you send it also to me.

Thanks a Lot.

My e-mail adress is: akocak4@yahoo.de
Fuad Dadashov
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Luca

Could you send me your presentation also

Thank in you in advance

fuadd@azdata.net
Tauqeer Syed
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Hi Luca,

If you could also e-mail me your presentations, i would be really greatful to you.

stauqeer@ajes.ae
Ali Vessali
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Hi Luca,

I would love to see your PowerPoint presentation.
My E-mail address is: avessali@ellisdon.com

Thanks in advance,
Ali Vessali
Forum Guest
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Hi,
is it possible for you to email the powerpoint slides to me as well please?
My email is hulahoops92@yahoo.com
looking forward,
Hamish
wilfred datinguinoo
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Kindly send the Powerpoint slideshow on my yahoo address
your message w/attachments that you’ved sent me has been quarrantined by our
mailsweeper.

Thanks,

wilfred
wilfred datinguinoo
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Hi! This is my email address

wdatinguinoo@CBIepc.com
we_wsd@yahoo.com

Thanks,

wilfred
Luca Basile
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Send me Your email, and I will send You some materials I prepared (courses in powerpoint) that will explain all these.
Forum Guest
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Wilfred,

Kabayan. give me an email, i could help you better through email. I’m not surprised that you got a not so detailed answers because your question is really very basic. Email me at sadeleon@tollways.net.

Hope to hear from you and extend you some help.

MK TSE
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wilfred, it’s a hard job especially you are in China.
Forum Guest
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No problem, It is always good to ask rather than keeping quiet.

Please get some fundametals first.P3 is a just a tool, but all the input must come from us first.

Best of luck

Dinesh
wilfred datinguinoo
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My apology for disturbing all of you for such a simple
question, like i’ved said I’m new in P3. I’m a welding engineer who happens to take over the job in Project Controls in our project and i find this site more helpful
in performing my job.

Thanks for your help

Regards,

wilfred

Bernard Ertl
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MK TSE
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Or read P3 menu to understand what’s forward/backward calculations which work out early/late start/finish and then work out total/free floats. There are some descriptions and formula in the menu.
Dinesh Kumar
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Hi,

This is very basic for planning to find out longest path and to determine the float.

P3 will actually do it for you automatically all these calculations.

Even P3 does it all for you,you should know this other wise you can not understand the significance of CPM and the float in your plan.

I suggest get some book on planning and read it.It is a simple concept and not difficult to understand.

Best of luck

Dinesh