Forward & Backward Pass

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

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!

Hi,

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

Thank you,

Marwan

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

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. 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

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

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

Total Float = LF - ES - DURATION + 1

Muhammad Anjum

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

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.

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

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