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Negative Lag.

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Dayanidhi Dhandapany
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Can someone highlight the advantages and disadvantages of using negative lag with clear examples. why client is against the usage of negative lag?

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Rafael Davila
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A required transition period might still be an issue, for any other scenario use volume of work lags unless the mere passage of time is the driving condition.

When updating the schedule time lags happen at a fixed rate no matter if predecessor works fastr or slower while volume lags are a function of predecessor progress. More often than not, lags are intended to be a function of predecessor progress.

2021-11-15-12-48-59

Imagine a three thousand activity network schedule with multiple relationships using time lags instead of volume lags.  Verifying and adjusting al remaining time lags upon every update can be daunting and error prone.

The same goes with Monte Carlo; job and activity non-deterministic durations as well as volume lags are calculated by non-deterministic production rates. If using duration type lag activities you might get thouthands of iterations that do not make sense. The key is on the source, the production rates.

Monte-Carlo-Volume-Lags

Zoltan Palffy
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Baseline Scenerio:This is a simple schedule built with 5 activities modeling proper lag versus a negative lag scenario. A baseline schedule was created as well as an update schedule to show the negative effects of negative lags when a schedule is statused and the data date moves. The baseline data date is set at February 1st. The negative relationship proposed on the Linkedin post “Finish to Start with a Negative Lag” suggested that activity B is tied Finish to Start negative 4 days ( FS, ‐4) to Activity A. This is modeled below in the section titled Negative Lag. In modeling a Proper Lag scenario that matches the dates in the Negative Lag scenario, activity B is tied to activity A, Finish to Finish (FF, 6). See graphic below  The graphic above shows that all Early Dates and Late Dates are perfectly in line and the float values match. Using Negative Lags in a “baseline”, non‐statused schedule will work fine. When that same schedule is updated with actual dates and the data date has been moved forward, negative lags will cause issues with forecasted dates and improper float values. Update Scenario:The Baseline schedule from above has been copied and none of the logic has changed. The activities in each section have been progressed exactly the same for Actual Start, Remaining Duration and Percent Complete. The Data Date has been moved from February 1st to February 15th. Since this is an update scenario Late Start, Late Finish as well as Baseline Total Float columns have been added to this layout to show what the Negative Lag has done to the dates in the ‘Negative Lag Scenario’. The layout below shows the tabular and graphical differences between the baseline and the update activities comparing both scenarios.  Conclusion:Looking closer at the schedule comparison the differences between Early Finish, Late Start, Late Finish and Total Float in the ‘Proper Lag Scenario’ and the ‘Negative Lag Scenario’ are glaringly apparent. Early finish of Activity B in the Negative Lag scenario is 4 days earlier now because the backward pass does not know how to handle the Lag of ‐4 days once the activity recorded an Actual Start date. The remaining duration of activity B in the ‘Negative Lag scenario’ begins at the Data Date instead of the logic and generates the forecasted end date of February 21, 2012 which is incorrect because the predecessor logic is no longer driving due to the negative lag. Basically the negative lag has overridden the existing logic.This is a four activity network and it’s clearly been affected by the use of a negative lag in one relationship. Imagine a three thousand activity network schedule with multiple relationships using negative lags. Float values would become completely skewed and forecasted end dates incorrect. The acceptable lag relationship is modeled properly in the ‘Proper Lag Scenario’ as it retains the correct CPM requirements for the forward and backward pass, and should be used for all relationships in a schedule that will be updated regularly                
Rafael Davila
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Rodel Marasigan
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Standard Scheduling Practice does not encourage to use lead or negative lag and shall be avoided if possible. A negative lag or lead is an overlap in the logic between two tasks – often it is used to represent a task starting earlier, with sufficient time allow some other work to happen. Lags cannot have a risk or uncertainty. It is likely that the negative lag represents un-necessary overlap, whose duration is uncertain. Consider replacing a negative lag with another kind of link that does not need lead or spliting the activity if necessary.

John Reeves
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OK everybody, is it now acceptable to use negative lags?  I have read these old comments and I think it is now considered OK.  I know for the last 30 years it was often in specs as "not alllowed" - but I think the reason that it was not allowed is now gone.  I think it pertained to not being able to go from P3 to P6 or to use 3rd party review software or something.  Personally I think ss and ff is better but some arguments in this thread make sense that it is ok.  Please advise.

Andrei Sannikov
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Hi Mike,

Excellent advice especially when you can convince your EPC contractor to go from L3 (that they have in their contract) to L4 or L5 where you would have "simple tasks with FS relationships". 

I don't like excessive lags at all and when I analyze my contractor's schedules I always run a report highlighting negative and excessive lags. My post was actually a comment on several posts I have seen suggesting to replace all negative FS lags with positive SS lags. All I am saying - play it by the ear look at each group of tasks individually. Even a negative lag can be legitimate in a certain situation.

I don't think you can put just one lag on a special calendar in P6 - the setting is universal for all the lags it can be either successor's or predecessor's calendar or 24/7 as you know and putting all the lags on 24/7 in a big network is probably not a good idea. You can add a curing activity (instead) and put it on 24/7 or 7 work days calendar and then indeed you might have a problem with float calculation in earlier versions of P6; looks like this glitch has been addressed in v. 8.1 that I am using now - will double-check in the office tomorrow.

Mike Testro
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Hi Andrei

What you have set down is the main argument of why SS FF lags with either + or - lags is fraught with problems.

What calendar do you put on the lags - curing must be 24/7 - which destroys the critical path if the lag ends in a holiday.

Use ONLY FS links to simple tasks that comprise one trade in one location and all your problems will disapear.

Best regards

Mike Testro

Andrei Sannikov
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I read very carefully all the negative lags related posts in this and other forums trying to find a logical answer to the same question I recently got from one of my contractor's planners. Unfortunately I could not find what I was looking for. Here is my four pennies worth on the subject: Schedule is a model of reality and lags are components of this model. If you need 28 days to cure your concrete then an FS relationship and a lag of +28 days between concrete and pump setting is perfectly OK; if you want your carpenters to start 7 days after your rebar crew has started then use SS and +7 days lag. On the other hand if you want to mobilize your hydrotest pumps shortly before your piping is compete (say 5 days) to make sure you flood your system on time then you need to use FS and -5 days lag - every thing else is illogical and would give you false results: if you  use SS with a huge lag your pumps might arrive either too early or too late depending on the actual duration of your piping activity; FF would not work either because I want pumps to be ready shortly before completion of piping. You cannot replace all FS with negative lags with SS positive lags - it all depends on how specific activities depend on each other in real life. If you want your schedule to produce true float and realistic forecast dates off course...
marcel cornelius
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Hi,

I also found out that negative lags are ignored bij risk analyses software such as Monte Carlo and Pertmaster.
Dayanidhi Dhandapany
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Paul & James,

thanks a lot for your good demonstration to clear some of my doubts that i had it in my mind.

Cheers!!!

Daya
James Griffiths
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Paul,

A nice expansion of leads & lags. As it goes to show, there are many ways of achieving the same or very similar thing...each with their own limitations, pros and cons.

Personally, I tend to avoid L&Ls, preferring to break-down the schedule in order to obtain tasks that are short-duration and retain relationships that are, as closely as possible, FS or FF with no L&Ls.

Unfortunately, in many circumstances we have a client who is using their own programme which attempts to mimic ours, but in summary. Of course, they want to insert summary-level logic that drives the (project phase)dates in accordance with the original baseline dates. This is where they often insert "FS+leads", owing to the fact that their summary tasks are of a significantly longer duration, but need to show the overlap between completion of Design and the start of Manufacture. However, on breakdown of the detail, in our own programme, there are no L&Ls. Everything is driven by FS/FF. The fact that the "overlap" occurs is because there are, say, 10 different sub-assemblies...and so we begin the manufacture of sub-assembly "A" as soon as the design is complete. At summary-level, this looks like a manufacturing "lead". However, should the 10th sub-assembly be delayed in its Design, the Design phase end date is also delayed which, in turn, at the clients summary-level programme, would invoke a delay to the start of maufacturing......blah blah, blah. The upshot is: be extremely careful if using L&Ls on a summary-level programme. If there is a genuine lag, such as waiting for the concrete to cure, insert the "curing period" as a genuine task. Quite how you deal with a "lead"...well..!!!

Cheers.

James
B.Eng.(Hons).AMIEE ACMI.
Paul Harris
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James

There are advantages and disadvantages of lags and leads, but there are some points I outlined in another discussion group outline, that may be interesting.

1. Most software puts the lag or lead (negative lag) on the relationship. You will find that some software such as Power Project puts the lag on the activity which is in my opinion more realistic. Then a lag or lead may be placed on the predecessor and or the successor activity and determined as a duration from the start of finish of the activity. This also assists in resolving resolves the calendar issue as the lead or lag is based on the activity that the lead or lag is assigned to. I am still in the process of learning Power Project but it has many appealing functions over other packages in the market place, especially for the building industry. When a lag is applied to a relationship, as in Primavera software and MSP, it is not clear from the schedule if the lag or lead is applied to predecessor or successor activity, but often obvious by inspection.

2. When a successor task may start half way through a predecessor task there are many ways of achieving this relationship.
a) A SS + Lag. The issue here is that the predecessor may have an open end if no FS successor is attached and if this has the finish date delayed then it will not delay the successor.
b) A % lag is available in MSP so when the predecessor duration changes so doe the lag, a little dangerous.
c) A FS + Negative Lag. At least in this situation the successor is delayed if the predecessor is delayed and a critical path is maintained.
d) A SS with Lag and a FF with lag. This is possible with Primavera products but not with MSP. An added complexity is introduced with this method and often one relationship becomes the driving relationship.
e) When leads or lags are NOT permitted the predecessor may be split into two, then the first half of the predecessor has two successors, the second half of the original task and the original successor. Again this is now adding another level of complexity with three activities and two relationships, instead of two activities and one relationship.

Options "a" and "b" are out in my opinion.

If a critical path and/or the ability to read the logic from a Gantt chart knowing that all the relationships are FS then one should use option "e."

On the other hand when the critical path is not important and the schedule is being used for say one week look aheads for site supervisors day to day management etc then option "c" in my opinion should be considered.

Option "d" does not add the visibility of "e", but provides interesting scheduling, but I would not normally recommend it.

So in summery if your client does not allow negative lages then the time an cost or preparing a schedule increases substantially.

Paul E Harris

BSc Hons, Certified Cost Engineer, PRINCE2 Practitioner
Director Eastwood Harris Pty Ltd

Paul E Harris
Eastwood Harris Pty Ltd, Melbourne, Australia
Planning and Scheduling Training Manual & Book Publishers & Consulting
www.eh.com.au


Dayanidhi Dhandapany
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Dear James,

Thanks for your lengthy explanation which is really useful.

Cheers!!!

Daya
James Griffiths
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Hi Diyanidhi,

Negative lag, or "Lead" is where a successor task can start before its predecessor has finished, even though they are linked on a Finish-Start relationship. Essentially, it enables tasks to overlap.

We use it where, say, we want the programme to prompt us to start writing Manufacturing Instructions as we approach the end of the design phase. The link would read something like this: "FS-20". This way, we can start the writing the instructions at the earliest possible time. If, however, the end-date of design phase extends to the right, it will automatically delay the writing of the Manufacturing Instructions. That’s just one very simple example.

Some people can use the "lead" to shorten the schedule duration by overlapping predecessor/successor tasks. If used properly, this is perfectly OK. However, by doing this, you can also use it to hide the true end-date, but without having to change any of the individual task durations.

It really is a double-edged sword. The same goes for the opposite of the lead. ie. the "lag"....Task B cannot start until 20 days after the finish of Task A.

If lags are not used carefully, you can accidentally lengthen the duration of your schedule, as well as the schedule appearing to be full of "holes".Moreover, you can use them to "pad" the schedule with contingency time. Because of this, and the "overlap" aspect of using a "lead", many clients do not like to see them, otherwise they become suspicious.

Hope this helps.

Cheers

James Griffiths.