Negative Lag

There’s lots of threads on this topic you can search for more detailed discusions, but I agree with your client.

Negative lag relationships are a nonsense for any detailed project plan. They will not (and indeed cannot) be followed on site, and hence do nothing but ensure your project plan does not reflect reality.

For example:
Activty B is driven by a FS relationship from Activity A, with a 2 week negative lag.
This means the guys on site do not know when they should start work on Activity B until Activty A has finished, by which time Activity B is already 2 weeks late.

In my opinion, negative lags are used by lazy planners who just want to make a pretty picture, rather than a schedule which will be used to drive the work.

Cheers,

G

As per CPM negative lag does not make any sense.I read in a book Engineers society i forgot exact named they strictly mentioned negative is banned in any schedule

Ban negative lag!

Breaking the Rules for the Typical Concept of Logic Ties

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

Why are sins always forbidden? A sin once in a while is not bad.

I agree that negative lag shall be avoided and used as rare as possible. But nothing shall be banned.

I do agree that negative lag doesn’t make sense on a situation which Gary depicted. Also I agree that many PMC’s do not accept to have negative lags in schedule.

However, I am putting forward an advantage which I found, in using negative lags. Consider the following situation of two activities A&B.
Activity A = Duration 10 days
Activity B – Duration 10 days (to be started once Act. A is 50% complete)

This could be scheduled with two types of links

Case 1: Act. B linked with predecessor link of SS+5day
Case 2: Act. B linked with predecessor link of FS-5day

In Case 1 we assume that Act. A will achieve a certain amount of progress within 5 days after the start date. But this assumption could go wrong. I mean Act. A might start in time but might not progress as per plan. However even in that case the schedule programmed with SS+5day lag will show that work front is ready to start work on Act. B, which is incorrect.

But if the schedule uses link as per Case 2, then the expected start date of Act. B will depend on the progress achieved on Act. A.

for me, if you ban negative lag, you should also ban positive lag or all lags.

negative and positive are the same, only they are opposite

"in every action, there’s an equal and opposite reaction"

I should have posted this in the other thread...

Sreejish,

Why agree that negative lag doesn’t make sense but then go ahead and find an excuse or a so-called justification for using it in a particular case?

You say, "This could be scheduled with two types of links"
Yes, but they would both be wrong.

You already found the flaw with case 1 and SS+5, which is that the start of the successor really depends on something actually being achieved on the predecessor, not just a passage of some days.

Case 2 sucks too, for a very similar reason, and more. You cannot know when you are 5 days from finishing the predecessor so you cannot know when to start the successor.
And again, the start of the successor really depends on something actually being achieved on the predecessor.

This then is what should be modeled, or else your model is not a good representation of the project.

Activity A1: Duration 5 days
Activity A2: Duration 5 days, A1FS0
Activity B: Duration 10 days, A1FS0

Hi Sreejish

Whenever there are mid task links then the logic will be lost when progress overtakes the mid link point.

If there is a logical reason why Task B can start 5 days after Task A starts then split task into two sections and use FS links.

Best regards

Mike Testro

Hi Trevor,

In case 2 as I had given above, we get a better control on start of act. B. Because as we start updating Act. A, we change the remaining duration of the activity, and this will push off the start of Act. B, if there is a delay in executing A.

If we have to model the program in the way as you said, we will have to avoid using any lags, positive or negative. I would love to do it, but you would know it very well, how difficult is it to model a program without lags.

I am not justifying negative lags. I was only comparing the use of FS+negative lag against SS+ positive lag.

Ideally i would like to have a program with FS links and limited positive lags.

Hi Sreejish

It is not difficult to do away with all SS FF links and use only FS links.

Send an email to

planning.services@xlninternet.co.uk

And I will send you a worked example of how an 8 storey insitu column - slab structure can be programmes using FS links in 20 minutes.

Best regards

Mike Testro

Hi Sreejish,
I don’t know the software that you use but if it supports seting more than one link between activities then you shall set both SS and FF links with 5 days lag.
In Spider Project you can also define volume lag - next activity will start only after 50% of preceding activity quantity of work was done.
Best Regards,
Vladimir

With so many restrictions the schedule no longer belongs to the Contractor. If the Owner wants to limit the use then let him make his own schedule and make the best use of the printouts. Toilet tissue is the most obvious but he can roll the printouts and be more creative.

Anyway, CPM is already so discredited that the Contractor no longer uses the tool; it is just a contractual requirement. You can manage any job the way the Empire State was managed, by today’s standards slow, just four stories per week, but finished without any delay claims. The cause of today’s claims is the CPM, so complicated after so many restrictions no one can get on an agreement.

CPM has become with so much granularity (otherwise you are lazy), that today you give your field supervisors a list with hundreds of activities each week instead of telling them just move that way and give me a required daily production with your crew. They spend the days trying to figure it out and throw away the paper, and then they have nothing.

BAN THE CPM

Rafael,


Your statement:

"that today you give your field supervisors a list with hundreds of activities each week..."

And there’s the balance to be struck in CPM, (or any other method) - trying to plan every activity down to the nth degree v coming up with a realistic but also useful programme to everyone on site.

Possibly it’s not the level of programme detail itself but the level of detail that is used to communicate what the programme is saying that is the real problem.

If by means of very restrictive specifications you are required to use a level of detail unnecessary for your management needs; it gets into your way. The end result is the schedule is set aside and field people do their best. No project manager is to sit down every week to summarize a 10,000 activities schedule, or over 100,000 activities schedule if a P6 freak is providing you with such a thing.

Believe me I have seen specifications that requires you to include 3 activities per submittal item (this adds to several hundred by 3), a coordination or quality control meeting prior to each activity, a maximum activity duration of 10 days, no lag therefore requiring at least an extra activity per lag, only SF relationships, to model your Payment Application with cost loading the network when this can be done at a fraction of the time with any electronic spreadsheet, no application for payment to be received until the updated CPM is also received, a TIA for every EOT request including rain, and the list goes on.

We had a NAVY job where we were required to submit every month 9 copies of the update in 2in binders plus the electronic copies, a small job, a 2.5 million dollars job. Fortunately the job got into problems with unforeseen asbestos disposal; the contractor issued a claim in the Federal Court against the NAVY and got awarded several hundred thousand dollars.

Sorry, if these are the rules of CPM then to the hell with it.

Best regards,
Rafael

Your answer is way too long for negative lag.
Negative lag means... if your asking questions about it you should not be here.
Simple.

Sreejish,
I beg to differ. It is easy to make a program which has only FS links and no lag (or maybe just a dash of + lag).
Actually, it is a lot easier to do than creating a tangled mess with weird links and negative lag, and the big bonus is that it is also readable and executable.
If, in spite of lots of advice to the contrary, you think your Case 2 is justification for negative lag, feel free to use it, but it will just make things harder for yourself and everyone connected with the project. But, hey, it doesn’t touch me so I don’t care.

Hi Trevor,
good project models in shipbuilding use almost exclusively FS links and are huge.
But in linear projects like pipeline, railroads, motorways construction using SS and FF links with positive lags is justified.
Regards,
Vladimir

By the way the Navy job had quite a lot of coordination activities to be performed the week prior to the start of any trade. We had two options:

A) To model the start/finish of the coordination meeting was with a SS/SF relationship and a negative lag.
B) To modeled the issue with a FS/SS positive lag using the Coordination Meeting as the predecessor and a as late as possible constraint.

We got a waiver for the use of negative lag, a waiver for the use of constraints was rejected. By the Navy being so picky about the use of constraints we had no other option. Here the use of negative lag resulted in not the best solution but the wrong one. Unrealistic float was being shown by not being allowed to use constraints. But because no constraints other than contractual constraint were allowed we had to go with option A in order to avoid issues and delays in the approval of the schedule.

Specs alone are not going to prevent your inspector being ignorant of all traps within the CPM, if you require a CPM then get a qualified reviewer. Let the contractor use the functionality that best suits his model, don’t be lazy and reject the incorrect use when appropiate.



Well this is another example of what’s wrong with negative lag but can you see what’s right?

Best regards,
Rafael