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Minus Lag

18 replies [Last post]
Thu, 2008-06-19 01:56
Jerome Atkin
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Guys / Girls !
I am working on a project where the planner is unaware / unsure why I have told him "no negative lag to be in the logic sequence". I am trying to find a document on why negative lag is bad planning practice.
I have been taught never to use minus lag in my programmes. “Forget the option is even there” I was taught.
The reason for this if you have: Activity (A) F/S – 10 with Activity (B) & activity (A) runs over it will push the start of Activity (B) out. As F/S Logic is being used.
If you would have used: Activity (A) S/S + 10 with Activity (B) then Activity (B) would start on the correct date as it is dependent on the start of Activity (A) not the finish of Activity (A).
Please don’t give me any smart arse replies like what if activity (A) was being delayed also !!! Or what about the open end (you should have S/S + F/F) I am trying to keep it simple !!. I know there are other reasons why minus lag should not be used but I would like to get your thoughts on this subject.
Lots of planners / software jockeys in the Middle East which I have come across use this feature a lot. I have seen minus lags of over 100 days. Which in my mind is lazy planning and they have no clue on what the proper sequence of events should be, so they just use minus lag to hold a date. It becomes really hard when assessing claims on their programme, due to critical path analysis etc.

I cannot find anything on Google so I need your thoughts, I will then write them up as this guy can read English really well, just not understand my broad London accent! He is from Korea.
If anyone has any papers on this subject or can point the way I would be really grateful or just post your thoughts here and I will review them and add them to my write up. I have started the paper but thought I would post here incase I missed anything out.

Thanks
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Replies

Wed, 2008-07-02 04:40
#1
Oliver Melling
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Gerome,

Maybe it is because, if planning a project you are documenting what you think should happen in the future.
By doing this you are not adding activities or logic prior to the date you are creating the plan.
Therefore, negative lag is not stating that you plan to do something in the past, it simply means you will start or finish task prior to the starting or finishing another task.

If you had a project to throw a party for new year, is it wrong to plan to ignite the new-year fireworks 2 minutes before the end of the year? (Assuming the fuse takes 2 minutes)

Should we be planning the months leading up to new year using FS logic?
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Wed, 2008-07-02 04:26
#2
Jerome Atkin
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I believe it wrong to use negative lag for the following reasons,
1) In certain software packages it affects how the critical path is calculated when scheduling as a result a wrong critical path is shown.
2) To start an activity in the past, I believe is bad planning practice. Better use of added activities or other logic can be used with the same effect, plus motoring the item.
3) It is used mainly by lazy people who do not understand – why they have used it – what effect it will have on the programme – people who want to fix the programme to a certain critical path.
I stand by my initial comment I believe it is wrong and bad planning practice. Only one person in this thread has come up with a possible use for negative lag. No one else has commented on why you should use not negative lag.
This is a grey area for planners – I am talking planners not software jockeys who use it as a quick fix without knowing why they have done it, they just know it is a function and the “result is the result”. "Talking about primavera" it affects the way the software schedules the critical path, so people can use this for their own ends to give a false state of accounts.
Do to the implications of using negative lag out weight the good reasons for using negative lag; I would avoid it as I have always done.
I have to adjust negative lag on a daily basis and it really gripes me why it was used in the first instance. I have seen minus lag from -1 to -250, WHY, not just once but 100’s of times in the same programme. When I ask why it has been used and this glum look comes over the person like “I thought it was ok – the button is there “In this whole thread only one person has given us a good reason on WHY it can be used. The majority have said it is bad logic. So why do I keep seeing it everyday !!! LMAO
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Wed, 2008-07-02 03:40
#3
Oliver Melling
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Some people believe that in ’theory’ negative lag is wrong, but nobody can ever say why?
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Tue, 2008-07-01 21:25
#4
Chris Koerner
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I try not to use negative lag in my schedules for a couple of reasons.

For me, critical path analysis and the correction of out-of-sequence activities just seems more difficult if negative lags are used in the schedule. I’ve seen negative lag used as a shortcut to replace using specifics about the work. I think the project is better served by using detailed activities so the negative lag can be removed. When negative lag is used to artificially shorten the duration of the critical path and show the project finishing on schedule, execution of the plan could be more difficult than it appears. Of course, if the schedule is poorly planned, it will be problematic to execute regardless of the use of negative lag.
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Fri, 2008-06-20 09:19
#5
Bo Johnsen
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Negative lag / lead: Logic or illogical, good or not good, can’t predict in the future what’s going to happen in the past that still would be considered the future from the present moment of time, etc......it really doesn’t matter. Some times contractor planners, especially those using MS Project, are just forced to use this negative lag due to the setup of how the programme nowadays a given such a high importance in the resolution of many delay claim disputes.

For planners using programming software were the feature of using both the SS and FF link between 2 activities, one could might call it "lasy planning" if they used FS minus lag instead, however, even in that situation there would probably be incidents where the FS minus lag would be more appropriate to use (unless producing a more detailed programme where a true FS-link then could be made).

However, many small contractors can’t afford the huge prices of Primavera and other "better suited" CPM software system. Therefore, they utilize MS Project and to "cover your own back" a FS minus lag most be utilized as combinations of SS and FF aren’t possible in MS project (at least not in MS 2000). An SS-link alone will cause an open finish end and a FF alone will cause an open start end. Not good should the project end up in a delay claim dispute.

So should the programme just be rejected because it contains FS-links with minus lag? No, because the system is in some places set up by the miscellaneous parties in construction projects/contracts in such a way that the use of FS-link with minus lag is simply a necessity, no matter if correct or not.

In theory, no, it is not correct to use the FS minus lag (which is probably also the reason why it is not allowed in most risk analysis software either), but in the practical world, yes, it is "correct" to use the FS-minus lag.


Regards,

Bo
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Thu, 2008-06-19 09:21
#6
Charleston-Joseph...
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Minus lag

or negative lag

is not good.

Logically, it is illogical

You can always find another activity to link the particular activity to remove negative or minus lag.

Let say, letter of award, then mobilization of sub-contractor.

there are a lot of ways to move around with this issues.

Don’t give them the 5% royalty or whatever monies.

Donate it to the ophans
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Thu, 2008-06-19 07:46
#7
Raphael M. Dua
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www.planningengineers.org

Raf
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Thu, 2008-06-19 07:08
#8
Jerome Atkin
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Do you have the web link ?
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Thu, 2008-06-19 06:37
#9
Joao Ribeiro
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I seem to remeber that the issue is addressed either in one of Gary´s notes at PEO site or in another one by Ronald Winter, also at his website.

HTH
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Thu, 2008-06-19 03:49
#10
Jerome Atkin
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But Trevor,

You must give the person a chance to tell you "WHY" it is correct. Even if I dont like it, I would have to agree with Oliver and I would let him pass "GO" with his reply.

The point being, if you know they understand the "WHY" they have used it. Then can it be wrong ?

I see many plans over here where they can not tell me "WHY" they just look blank !
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Thu, 2008-06-19 03:45
#11
Jerome Atkin
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Oliver,

Please dont get me wrong.
I agree with your example. In fact a very good reply.
Also agree with you new post and was silly of the reply i posted. We all get quotes etc before hand.
For me thought i still dont like minus lag.

But you reply will be one "FOR" the use of it. This will be an example in my paper i liked it so much !

Thanks for the reply
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Thu, 2008-06-19 03:45
#12
Trevor Rabey
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Well that’s right. If it’s for real then it is still hard or impossible to interpret without an explanation which is never provided.
No explanation is ever provided because it is nearly always deliberately intended to be deceptive or is done by someone who doesn’t know what they are doing or what’s wrong with it, or could be wrong with it.

Even if negative lag can be in the plan and make the bars look like they are in the right place, it causes huge conflicts with execution according to plan and with tracking.
So many ways it is wrong and only so few or none where it is not, so reject it, full stop.
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Thu, 2008-06-19 03:42
#13
Oliver Melling
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Gerome,

Project i have worked on ussually get quotes from sub-contractors at the tender stage of a project to help form the tender price.
The one week in my example would purely be for mobilisation.

Oli
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Thu, 2008-06-19 03:38
#14
Jerome Atkin
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Guys thanks for your input.
This is not for a book of anything, i will post it once written on here !
I would expect a good answer for the use of minus lag.
To be honest I don’t like it (just the way in which I was taught). I still would have put Oliver’s logic as S/S and monitored the progress of the “Strip-out” to ensure it was far enough ahead to be able to bring in the other sub contractor. In some parts of the world trying to source contractors and have them approved does not take one week. All sub contractor here have to be vetted and it take F”ing ages !! So for me this logic would not have worked (different projects – different countries).
My point was that people use this tool (if you call it that) to bypass true logic, without a good reply (Like Oliver’s) to why they have used it. They use it to hold dates in place or to show a different critical path, which is untrue.
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Thu, 2008-06-19 03:28
#15
Oliver Melling
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Same goes for me, except i want 5% of the royalties!
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Thu, 2008-06-19 03:21
#16
Jerome Atkin
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I see your point.
as in this case it was dependant on “one Week before” I suppose the logic would work. If the strip out was running behind then you would not be bothered by this (as you 1 week lead time) would simply move along with it(strip out)
Good answer to my question.
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Thu, 2008-06-19 03:17
#17
Trevor Rabey
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This is easy.
You had good advice.
Does a swimmer jump off the blocks 0.5 second before he thinks the gun will go off?
Wouldn’t it be gambling, rather than planning, if he did?
How do you know when to start a task if the trigger for it will go off in the future?
You can start on the assumption that predecessor task will finish on a known date in the future.
But how often does that happen, and what if it doesn’t?

Try searching Google (the web and the groups) for "negative lag".

Don’t assess their claims; reject them.

If you use my stuff in your book, make sure there is an acknowledgement.
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Thu, 2008-06-19 03:13
#18
Oliver Melling
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Gerome,

If for example, a PM needs to mobilise an asbestos removal sub-contractor 1 week before the stripout of a building (due for demolition) is completed, how is a FS -5days logic sequence wrong?

Oliver
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