Negative Lag

Believe the planners shall avoid using the negative lag with FS relationships. Try positive lag with SS relatioship and close the open end with reasonable positive lag FF relationship.



As a layman, owner might feel that negative lag with FS relationship can mislead the schedule or the logic of work is not retained.

Bernard, thanks for the link to the interesting discussion at PMI COS.



I think the simplest explanation (other than software functionality) for avoiding negative lag (in any type of relationship!) is that it triggers an activity start or finish on the basis of an event that has not yet occurred. That necessarily increases risk, as the triggering event may NEVER occur! That said, I have occasionally used negative lag in situations where I felt that any other modeling method represnted an even greater distortion.



As to the discussion at PMI COS, I’d just mention that one of the drawbacks of using any complex dependencies (SS, FF, SF) and/or lags is that it can make the task of manually computing critical path DRAG very difficult.

Negative lag is allowed only in certain cases.if the completion of an activity is delayed by its predecessor then you can never start the activity and hence the activity goes out of sequence. Eg:Commisioning of Substation if Panels are not installed.

Hi Bill

Strange Rule of Thumb.



Maybe we should ask why we us Lag. We use lag positive or negative if the schedule is not broken down to enough detail. Unfortuanatly when we start projects engineering has not been done we do high lvl schedule (Lvl 1/2/3)in this case lag is used.

Once detail is developed these lags normally drop away.

To have a rule no -lag is allowed I think is unrealistic. (You can apply a rule in lvl 3/4 schedule no - lag is allowed only if this is part of your definition for a lvl 3/4) In planning we use all kinds of Methods and if we stop using negative lags we stop using a planning method so then we can just as well stop planning.



If you ever wondered the best reason for negative lag is due to the rate of progress. This minimise cost P & G. for following contracts.







Cheers

One more reason to not allow the contractors to use negative lag in general that the most of the time it is used for manupulation to meet the target dates initially.

Here’s a similar topic being discussed at the PMI COS:



Negative Lags of Finish to Start Relationships



Bernard Ertl

eTaskMaker Project Planning Software

I just looked into the Primavera Knowledgebase and found that there is a small bug with Monte Carlo and negative lags on finish-to-finish relationships. The successor activity finishes on the correct day, only one minute earlier than it should. I can’t recall seeing a finish-to-finish relationship with a negative lag on any schedule that I have ever dealt with. Otherwise, I did not find any other issue mentioned. By the way, is that product even for sale anymore?



I did not see any such problems with the other Monte Carlo software sold by other vendors. Is this the only issue with negative lags?

Bill,



You say the risk analysis cannot be done on a negative lag? This is very interesting. Can you expound more on this? Thanks in advance.

Hi Guys,



What is the relationship? FS, SS or FF, the use of negative lag is only acceptable in the case of FS. Some programs such as pertmaster, does not even accept 0 lag in the instance of SS or FF, when risk analysis is carried out. What is the clients reasons, for objecting to negative lag? Is it due to a problem during risk assesment or just a personal preference?



Regards





Philip

I am not sure why the owner does not allow it but I THINK it has to do with the detail. If you have a high lvl schedule you apply - Lag. If you get to a detailed schedule - lag dropp away becuase of the detail. So all i think the owner is telling you if you need negative lag the activity is not broke down to enough detail.



Hope this helps