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Earliest Start of an Activity based on a group of predecessors

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Rami Al Haddad
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Was wondering it it is possible to do this in P5/P6: 

   To schedule an activity such that it's early start date is the earliest finish of a its of predecessors (many)!

Many Thanks,



Rami Al Haddad
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Thank you all for your replies


@ Sharif: my reservations on FS w/ -ve lags is (1) negative lags are not good practice and they seem to get people jumpy and taken as a sign of tricky business (2) what happens if an activity was updated such that its remaining duration is more than the amount of lag u've originally accounted for!


@ Vladimir & Rafael: Thank you both for you insights,,, I've worked out something with the spirit of what you both described and it works fine,,, wish I get the chance to get to know Spider Project more,,, that'll be my 2011 resolution!!!


@ Ronald & Stephen: Ronald, Stephen gave a wonderful example there which i believe cannot be matched in the construction industry. The cases that I have encountered; as bizarre as they might be:  

1) The NTP was conditioned by 3 different events; whichever occurs first.

2) The decision of the client on whether a PS package is to be constructed or not must be given following Schematic Design / System Specification and  prior to Preliminary Design. Schematic Design comprises 3 different packages and the decision must be taken when any of the packages is successfully completed.


Best Regards,


Stephen Devaux
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Roalnd Tannous wrote:

"Why are you exactly trying to implement such a relation. It sounds counterintuitive.

Could you please elaborate on the situation at hand?"

Roland, this is actually a fairly common logic link in pharmaceutical development projects. You fund two drugs intended to effect the same cure.  Whichever first demonstrates successfully completes animal trials (and show itself to be effective), you kill the other one and proceed with the development of just one.  Otherwise, you might wind up competing against yourself.  I believe in Microplanner (which, as Rafael correctly points out, is one package that supports this) it's called "reverse logic".  

Fraternally in project management,

Steve the Bajan



activity shall start if any preceeding activity finishes. So It does not matter when other activity will start and finish.

Of course resource (or material) production and consumption is more effective way to simulate this case but not with P6.

I remember about conditional links, be patient.

Best Regards,


Rafael Davila
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All advanced scheduling software provides for resource creation and consumption, I believe Asta and Micro Planner both can model the creation of resources similar to Spider Project, therefore P6 should have similar functionality.

Because I do not have P6 perhaps by showing you how I model this within Spider Project, maybe you can use similar methodology within P6.

First create a dummy resource for every activity you want to check for a group of predecessor. Assign 0 availability at the project start and make predecessors activities to create one unit of such resource, in this case to create such resource at the end of the activity. Assign the successor the amount of resources equal to the required number of predecessors. Then when you run resource leveling the activity will be scheduled to happen after there have been created the necessary resources or after any other logic dependencies are satisfied. The following is from a sample job using Spider Project, just mimic this using P6, is very simple procedure.

Start after Xno of predecessors


As per your suggestion, what will happen after first activity finishes and the successor is using all available resources, then no other such “predecessor” (not any longer “predecessors”) will be scheduled after the second activity finishes. Am I missing something? What will happen if the first predecessor finishes but no other have started, I believe here the term predecessor refers exclusively to the finish event of any activity on the group. I suggest use resource creation, is easier.

I believe without resource production and consumption functionalities this cannot be correctly modeled using resource leveling. Also I am waiting for the promised Spider Functionality for formula links. I would also like to be able to model such issue using logic links that are toggled on and off using conditional formulas. I forgive but do not forget.

Best regards,


Roland Tannous
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Why are you exactly trying to implement such a relation. 

It sounds counterintuitive.

Could you please elaborate on the situation at hand?


 RDCMP can do this.

I suggest to assign the same resources to all N predecessor activities and the successor activity.

If total number of assigned resources is N and the project is levelled the successor will be delayed until some predecessor will finish.

To be sure that successor will start after the predecessors link them not with FS but with SS link.

There is easier solution with Spider Project but you asked about P6,

Shareef Abdul Azeez
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Yes, it is possible, but you will have to use a FS with -ve lag and among all the predecessors, the  driving predecessor will determine the start.


I have asked to use FS -ve lag having understood that you require the Finish of a Predecessor to be the Start of the successor (meaning the same date & exact timehaving scheduled your datadate to the end of your working time). Can you explain a situation where you are going to use this????

Another solution would be to use a SS volume lag on your predecessors which is not supported by Primavera.



Best Regards

Shareef A Azeez