Negative Lag

I think using a -ve lag is not a logic relation.

ex. The Predecessor has to achieve a 100% progress to allow the successor to Start in an earlier date.
That will specially appear clearly while updating.

We have also experienced some unexpected resluts when using those lags in P3 specially when incorporating in complicated schedules containing many relations and constraints.

A 100% completion for an activity is not always acceptable by some Clients which will lead to an out of sequence if the successor starts.

Tarek

I dont support -ve lag appear on any programme. My view is non-zero lag is an activity. Add one activity rather lag. Further, duration can be control by calendar in P3 but lag is not.

mktse

I think using negative lags with a FS relationship is perfectly acceptable.

Just because you cannot do this in ADM is not a logical reason to not do it in PDM. Lags in P3 most certainly do use work calendars; the work calendar of the predecessor activity.

The only alternative to a FS relationship with a negative lag is a SS relationship with a positive lag. Very often, using a SS relationship with a positive lag is misleading and incorrect.

What if the predecessor activity experiences a delay? With a SS relationship, the successor activity can still go ahead an start. A FS relationship with a negative lag would have prevented the incorrect start of the successor activity.

If the successor activity is supposed to start slightly before the completion of a predecessor activity, then you have just described a FS relationship with negative lags. But the way, it is FS relationships with positive lags that you should be worried about.

Tarek,

Could you elaborate on what kind of unexpected results you got when using negative lags on a FS relationship?

This situation can be scheduled with straight FS relationships if the predecessor is split into two activities/tasks so that the first task links to the split task and the successor task. Ie.:

-- Original --	|	----- New -----
A[10hs] -(- 2)-> B[8hs]	|	A1[8hs] --> A2[2hs]
	|	&
	|	A1[8hs] --> B[8hs]

Bernard Ertl
InterPlan Systems Inc. - Project Management Software, Project Planning Software

Bernard

Splitting the activity is the perfect solution for such a case.
You will only need to convince all concerned parties (owner - consultant - contractors etc.) why you added one or more acivities.

From what i remember (it is more than a year now) is the following:
- The main Electro-mechanical contractor used the -ve lag to Start his activities linked to our Civil works.
- At the time of the monthly update our Activities were not a 100% as per the Owner/Consultant review.
- The result was an obvious out-of-sequence as the Electro-mechanical activities Started.

I am not able right now to detail the situation technicalities in P3 but i will try to see what i can do.

My point of view is that how do you Start a Successor before knowing that your Predecessor has complied with the relation rules.

Tarek

Tarek, I agree with you 100% that a negative lag on a FS relationship is a logical fallacy. You cannot start an activity in the past based upon the completion of an activity right now. At least, until someone invents a time travel machine.

I am curious with regards to actual consequences experienced or observed though. I will appreciate your follow up whenever you have the time.

Bernard Ertl
InterPlan Systems Inc. - Project Management Software, Project Planning Software

You think that splitting the predecessor is the answer to negative lags? What are you going to call the first activity, "Electrical Rough-in, Everything but the last 3 days"? Then I suppose the second activity will be called, "Electrical Rough-in, the last 3 days".

Then, how are you going to status "Electrical Rough-in, Everything but the last 3 days"? When are you 50% done?

In reality, this technique does not work well and I wouldnt allow the Contractor to submit such an odd duck.

Hi Ron,

I would not suggest your proposed descriptions. To me, tasks should be logical and discrete activities. I am not an electrician, so I would hesitate to offer a specific example to replace your proposal, but I have always found that tasks can be subdivided into further detail with a little thought.

The key is to identify what the core relationship is between the successor and the predecessor. Why should it start 2 hours before the other one ends? Why not when the predecessor starts? What is the logical antecedant that must occur before the successor task may start? If there is no logical reason, then using the FS w/negative lag is just painting a picture and not really critical path scheduling IMHO.

Bernard Ertl
InterPlan Systems Inc. - Project Management Software, Project Planning Software

Hi everybody,

Ron there are always many acceptable solutions in such cases.

The easiest in my opinion is to split the general constuction activity to identify more areas
(ex. conduiting in conc. slabs - exposed wall conduiting - conduiting above false ceilings ... and so on)

In that case you are not introducing any new tech. description - just locations.

The trick here is do you really need that or is it just complicating unnecessarily you schedule.

Tarek

I have often come across negative lags when examining plans.
Upon questioning 90% of the time it is when a barchart has had logic added to make it look like a critical path plan!

My answer in all cases is to get to understand the logic that is "underneath/ behind" the programme. This can quite often bring revalations to the person who originally put the links in as their knowledge of the job increases.

I agree that by adding greater definition/ extra activities that the logic links can be defined so that negative lags need not be used. To me negative lags are in the main a result of sloppy planning or where a planner is unable to extract the necessary understanding of the job from those that he is working with. They are therefore not necessairily the fault of the planner, but higher management, for letting a job proceed where a full understanding of the tasks to be undertaken is not known/ appreciated or defined.

Hope this helps the debate

Guy

[duplicate post deleted]

Hello Guy,

I agree with your comments, yes certainly it depends upon the logic behind the schedule. I too come across some building schedules from sub-cons, showing that finish to start relationship with negative lags. for example in a building project a floor can be divided into 2 zones, each zone can be completed in 7 days, hence sub-cons usually tempted to show Structural works Level 1 linked with Structural works Level 2 by Finish to start minus 7. [i.e. Assuming that after the completion of Level 1(Zone A) structural works formwork can be started for Level 2(Zone A) structural works]. May be due to lack of time in preparing detail schedule, they are reluctant to show detailed activities, which would create some problems to the critical path certainly if they are not finishing any one of the zones due to some problems and proceeding other zones at different levels of a building(imagine if we have 7 or 8 zones in any level).

Regards
Daya

Hi All,

What is the result of removing negative lags on the activities that were already completed?
I am asking because I took over a schedule that has a lot of negative lags and the client specs don’t want those negative lags.I am using P6. Thank you very much.

Regards,
Joel

Joel,

Activities that were completed are already fixed (with actual dates), removing lags and relationships will not affect it (when running the schedule).

I suggest a recovery plan and a proposal to re-baseline (copied from Carmen).

Hi Anoon,

Thank you for the enlightenment.
At this moment recovery schedule is not in their mind because the schedule shows we’re not behind schedule. It only bothered me to think that when the time comes (project finish) to submit the as-built schedule; they’ll find out these negative lags and it may cause rejection from them. That’s why I am thinking of removing lags on the completed activities. Everybody here is new in this scheduling software it seems I am just here because it’s required by the client to have a scheduler who uses primavera. They prepare a small piece of paper made in excel to tract their progress. Thanks a lot.

Regards,
Joel

I just found this:

Breaking the Rules for the Typical Concept of Logic Ties

http://www.htcprojectcontrols.com/TTB2004-2.pdf

If you like free goodies then from this site you can download for free a file comparison software for P3, I have not tried it yet, new to me. Not as straightforward as Digger but free.

http://www.htcprojectcontrols.com/Software.htm

Enjoy ! ! !
Rafael

All
This is exactly the types of discussion we should be having in the forum - none of this ’send me a copy’ nonsense
Nice one!!

Shimmo