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Volume of work lag in Forensic Analysis

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Rafael Davila
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The AACEI RP 29R-03 2011 on the modeled substractive methods provide for extraction of entire activities or a portion of the as-built durations representing delays or changes from a network analysis model representing the as-built condition of the schedule.

If two overlapped activities with original duration of 10 days overlapped by 5 days lags are reduced to five days then common sense tells us that lag must also be reduced to half.

This reduction is not automatically performed by many software as they only model time lag. This time lag yields wrong modeling when true lag is volume of work lag as it is most common in the construction activities. You keep a volume of work distance that is active as long as the predecessor is not finished, volume lag correctly models this while time lag is no longer active after it is consumed by time. Time time lag does not consider in any way the volume of work required distance.

I believe if subtractive models are restrictive, always in favor of the owner why add more with a wrong model using time lag instead of volume of work?

In the following sample job pacing is being extracted and the correct model, the one using volume of work lag yields a shorter collapsed network by a duration of 3.75 days.

Is very simple but the AACEI RP is protective of the limited functions of some software, they mention Primavera, although not as much as before, but make no mention whatsoever of this modeling functionality that is available in Spider Project and that can yield better models. They do not prohibit the reduction in lag but require any reduction in lag be justified when some software can do it within the correct modeling technique, under such models there shall be no need to justify one by one each due reduction in time when there is no reduction in volume lag.

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I wonder how many of you have been in need of reducing lag in your collapsed models and how you handle the issue when it becomes frequent? Do you need to justify a better modeling algorithm for every occurrence?

Please note I am not saying time lag is always wrong, concrete curing is a very correct lag when modeling concrete curing, a lag with 24/7 calendar while successor and predecessor activities might have each a calendar of their own.

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Rafael Davila
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Mike,

The issue here is not about the selection of a forensic method is about rationalizing the associated reduction in lag after extracting some duration of overlapped activities.

Time Lag modeling is fixed and wrong, it is better modeled with the use of Volume of Work Lag.

One of the key points is that it is not merely at times but at the overwhelming majority of cases. It makes no sense at all avoiding the issue. 

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It does not matter if you measure plaster not by the CMU but by surface area. Only double links will look for the volume of work on the successor, on simple links predecessor sets the distance.  

Now I have a case in which the contractor increased the size of some crews for critical activities to the point when idle he would use the excess production capacity to work on non critical activities. The as built durations for these non critical activities must be reduced, but also the lag, here AACEI is of no help, but the contrary. Because there was a change in sequencing of work due to the delay in one building the as-built non critical activities become critical in the collapsed as-built and their lag is on the collapsed as-built critical path.

In our construction planning there is need to overlap many activities (and I mean many, many, many...), but time lag is wrong, the justification always relies on a needed physical distance modeled with volume of work lag but not an arbitrary time that expires, volume of work lag is non-existent in American software and is also avoided in our forensic protocols.

Ironically if you increase duration of activities due to a reduction in productivity the time lag model will favor the Contractor. Forward looking methods (as-planned) are as weak as backward looking (as-built), and perhaps worst. For the selection of methods I find the AACEI to be a great guide.

By the way, unless you change logic, updating can break Time Lag only once while for Volume of Work Lag the updating can break or restore the link more than once.

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Best regards,

Rafael

Mike Testro
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Hi Rafael

In real life forensic situations nobody remembers why a task was delayed or why it started when it did and there are no records to prove it anyway.

It is just impossible to put logic into an as built programme because there is no evidence to say why it should be there.

The example of lags in your chart is pure theory and does not apply in the real world.

As an aside PowerProject has the facility for volume lags as well as time lags - each just as useless.

Best regards

Mike Testro

Rafael Davila
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Mike,

The AACEI falls short with providing examples of the procedures, it mentions Enhanced procedures but misses too many. One of the Enhanced procedures it misses is the application of Volume Lags or perhaps the necessary adjustments to Time Lags when in reality the model calls for Volume of Work Lags. They are not the same, they are not modeled equal. The lack of mentioning of this real need not only promotes substandard modeling but denies the opportunity to make the adjustments to those who are using substandard software.

Take for example:

A construction contractor when scheduling the installation of masonry units and then the plastering of the units, he knows there is a real space constraint and that he must keep the brigades at a physical distance. This distance in the absence of Volume of Work Lag is modeled using Time Lag. But this is wrong.

Say a separation of 4,000 CMU units is a reasonable distance. In the absence of Volume Lag he will fill this void by using Time Lag. If the estimated production is 1,000 unit of CMU per day this translates into 4 days of Time Lag.

If after 4 days the realized production is 2,000 units this means he cannot start the plastering yet, but the Time Lag model will say he can because the 4 days already elapsed. It does not take a Guru to realize this is wrong modeling.

If after 2 days the realized production is 4,000 units this means he can start on the third day with the cement plastering, but the Time Lag model will say he still have to wait for another two days. It does not take a Guru to realize this is wrong modeling.

There is no need to penalize hundreds or thouthands of contractors that under this wrong modeling pay this omission with their money on their claims. The problem is that American scheduling software lack this functionality. Why the omission by the AACEI?

The AACEI does not explicitly prohibit the use of Volume Lag, simply ignores them. Do not even provide for guidance on how to adjust Time Lags as to make up for this difference. They require for the analyst to justify each reduction (or increase) in Time Lag, one by one and provides no guidance with regard to this everyday occurence. In the Collapsed-As-built they allow to extract pacing duration but no guidance is provided for lag associated with pacing.

Yes Mike, I am with you regarding wrong use of Lag, but there is an abismal difference between Time Lag and Volume of Work Lag.

How in court can you reason use of lag to keep the distance and not respect the meaning of it in your modeling?

To say there must be an arbitrary time separation, instead of a physical and logical reason is wrong, but even worse is when your own model does not respect the distance constraint.

Best regards,
Rafael

Rafael Davila
User offline. Last seen 23 hours 26 min ago. Offline
Joined: 1 Mar 2004
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Mike,

Don't you get it, the real problem is with time lag, there is no such problem with volume of work lags. Perhaps your Asta PP cannot handle this and same as the AACEI you pretend volume of work lag is non existent.

I can agree with you 99% of time lag is wrong because 99% of the time it shall be modeled using volume of work lag, they are not the same, they are very different models.

If you read in between the lines AACEI dislike time lag but do not know about volume of work lag or pretend it does not exists.

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As a matter of fact this is the real world and most frequently I use time lag with the understanding it is not the best option, it is what my clients with their limited exposure know, it is that our CPM scheduling culture is archaic, a reality I have no problem at all to expose.

Best regards,

Rafael

Mike Testro
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Hi Rafael

In your latest thread you have supported my case that SS Links with Lags are an abomination and should be banned.

A task should contain one trade in one geographical location where the trade can work on its own.

So when the masons have finshed laying blocks on Level 3 Zone 2 they can move on to the next zone and the next masonry task.

The plasteres then move in behind them (assuming this building has no need of electricty or plumbing) and they are all linked FS with no lags.

Plan it ptoperly and you will have no more volume or time lage problems.

For the contractor each project is a continuing game of snakes - no ladders on the board at all.

Best regards

Mike Testro

Rafael Davila
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In construction modeling there are Ladders and there are Ladders, not all are created equal nor modeled in the same way.

We have:

  1. Ladders that are driven by time lags, this is how many software model this type and other types in the lack of more functionality. Many have questioned the use of time lag and they have their point mainly because in reality time lag is not a good model to use in lieu of volume of work lag when it is the actual condition. Some have said the only time lag they recognize is curing operations or the like. My ballpark estimate on these type of ladders is that they happen in about 0.9% of the cases but wrongly used in 99%.
  2. Ladders that are driven by volume of work lags, this is where most construction ladders do happen. Take for example masonry and cement plaster of masonry units. The contractor cannot install 30 CMU units and then cement plaster as soon as technically it can be done, there are practical limitations like use of space, you cannot perform masonry installation in the same room you are cement plastering, you must continuously keep the distance. The key is in modeling the keeping of the distance, time lags are incapable of continuously keeping the distance, once their initial time expire they are no longer active, on the other hand volume of work lags are always active and correctly model the keeping of the distance. My ballpark estimate on these type of ladders is that they happen in about 99% of the cases.
  3. Ladders that are driven by a continuous feed and whose predecessors and successors have a circular relationship. Many say this is a need exclusive of a factory production line. But it do happens very rarely in construction. So rarely it is difficult to imagine an example. My classical example, based on my own limited experience, is the construction of a sea breakwater where you make a bit of breakwater, use this bit to place the clamshell that will excavate loose sand and then use the bit of breakwater to dump rock on the recently excavated area. This is different to the modeling of masonry and cement plaster as you can finish masonry, the predecessor, without doing any cement paster (the successor). Here you cannot finish the excavation of loose sand (the predecessor) without finishing some portion of the rock back filling (the successor). My ballpark estimate on these type of ladders is that they happen in about 0.1% of the cases.
  4. Ladders that are a combination of any of the above. My ballpark estimate on these type of ladders is that they happen in a negligible number of cases.

If your software is incapable of modeling volume lag and instead resort to the use of time lag then the correct procedure is to manually adjust for the lack of a good model.

My contention is that the AACEI by omitting in their document mentioning this is making it difficult for the forensic analyst to make a very valid claim, or shall I say to do the right thing. It is not that they prohibit to do the right thing, it is that they are not doing enough to provide good guidance here. Their omission of this as an enhanced procedure have no excuses.

In the lack of any comparable reference I am adopting the AACEI RP and I am loving it, but at times it falls short and I feel the urge to challenge it. PP can give me access to other experts whose opinion matters to me.

Rafael Davila
User offline. Last seen 23 hours 26 min ago. Offline
Joined: 1 Mar 2004
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Mike,

Here at some point, specifically at the who to blame responsibility you must exercise educated judgments. These judgments are a required part of your presentation as mere facts presentation is not enough .

You said "the opposition's guess is just as good" - here it is not enough for the opposition to merely present opposition he must also exercise educated judgments and then at the end the Judge judgments will follow, it is not enough to call the other party as wrong or subjective.

Take for example an activity planned to take 2 days, it started on day 10 but ended on day 30, this initially yields an As-Built duration of 20 days. The collapsed duration might be 3 days because there was pacing. With methods such as one of your favorites you can make an educated judgment call. Say it is valued on BOQ and substantiated by cost references to be $375 then labor cost can be estimated to be $120 perhaps 2 man-days depending on the applicable labor rates. The collapsed duration is around 2 to 3 days.

Even the final judge ruling is a subjective one, but an educated judgment.

My issue here is not about the selection of a forensic method but on the application of volume lags, the issue can be extended to any of the possible modeled methods and its variations. My issue is on applying the correct math to lag when they are in reality volume lag and not time lag contrary to the case of Concrete Curing, one of the few cases where time lag have real meaning.

But curing time is a finish to start relationship and CPM practice calls FS lag as bad, it shall be represented by an activity. Maybe there is no valid place for time lag while volume of work lag is valid, a lag that is modeled differently to keep its true meaning, it is not merely a changed name.

http://youtu.be/Wl94YyipoWA

Best regards,

Rafael

Mike,

you wrote:

The duration of an as built task could contain time related delays as well as volume of work.

How are you going to adjust the lag to compenstae for the different events?

 

It is easy - create two links, one with volume lag and another one with time lag.

And add all other logical constraints creating additional dependencies.

 

About Why:

Spider Project keeps project archives (project versions created with each schedule update) as separate files that can be analyzed together. We strongly recommend to link (or include in these files) project versions with the Meeting Minutes where plans for the future week were approved and why, current problems were discussed. So you have project history and you can easily find what activities were critical at any moment, what problems and risks were discovered and discussed, what decisions were made and why.

 

Best Regards,

Vladimir

Mike Testro
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Posts: 4418

Hi Rafael

"the weakness relies on the analyst performance not on the method." 

No - the weakness stems from the detail in the as built records.

If you are going to do a colapsed as built then you have to know not just "who did what when and where" but also WHY.

Anything less and without any evidential value the analyst is just guessing and the opposition's guess is just as good.

That is why the method id frowned upon in UK jurisdiction.

Best regards

Mike Testro

Rafael Davila
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Mike,

In our courts in some jurisdictions non modeled methods such as mere bar charts have already being rejected many times as they are considered not enough to prove the relationships that caused the delay.

When you do not have updated schedules and not even an approved baseline a Subtractive method is your only option.  Our courts differ from yours substantially. Even Puerto Rico courts are different to mainland USA, our system as well as in perhaps only a single mainland state accept the English and Spanish codes. Here our judges rulings are not limited by common law.

I am well aware of your statement and in Caletka book it is also expressed, I see it more of a warning about taking due care because these methods require higher standards of performance by the analyst, the weakness relies on the analyst performance not on the method.

The issue is about options when non-modeled methods have been consistently ruled out on our courts. In your courts seems like non-modeled methods are accepted and that there is preference of certain methods (Modeled), kind of similar but other selections. I must play within our rules and consider the audience. Perhaps in mediation or out of court negotiation I would consider use of Observational as opposed to Modeled methods.

Still British experience to me is good reference, just do not apply it literally on this side of the pond.

Take a look at the following AACEI table, of eight modeled methods four, 50% make reference to Collapsed As-Built, here Collapsed As-Built is considered valid.

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... 50% does it means just half?

Here a method that dynamically considers how the original logic changed is thought to be more forensically accurate than that which statically relies solely on the baseline logic. All As-Built methods take into account as-built logic, perhaps all better than those who rely solely on static As-Planned logic, though not better than those who rely on As-Plannedlogic at time of impact. But we lack the data to sustain planned-logic at time of impact. In our case the selection is not that difficult, got to be one of the four implementations that rely on a Collapsed As-Built.

Mike Testro
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Joined: 14 Dec 2005
Posts: 4418

Hi Rafael

You are talking about the colapsed as built method of delay analysis which is always a problem because you have to put retrospective logic into the as built programme which is always objective and easily challenged.

The duration of an as built task could contain time related delays as well as volume of work.

How are you going to adjust the lag to compenstae for the different events?

My advice is don't bother with Colapsed as Built method.

Best regards

Mike Testro