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The Negatives of Positive Lag

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Emily Foster
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Are positive lags acceptable? We look at why positive lags in the schedules should be limited http://ow.ly/HrzP30cHKc6

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Rafael Davila
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RELATIONSHIP FLOATS IN P6

 photo P6RF01_zps2bugge8d.jpg

https://www.linkedin.com/pulse/relationship-float-viewing-primavera-p6-santosh-bhat

While the calculations are there maybe in order to view them in a links table if using P6 you might need Logic League Extended Relationships Database.

http://ronwinterconsulting.com/Extended_Relationship.pdf

In complex schedules with thousands activities and many more links it is impractical to navigate different screens to get these values.  Do not dream the activities table or GANTT diagram will make it.

At the end of the day in order to tame logic you will need to look at the relationships table.

Rafael Davila
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LAG DRAG:

Lag Drag is the result of correct contiguous model calculations; it is not a calculation issue but a lack of understanding. 

Contiguous model is more efficient in terms of the individual activity.  Unless the activity is on the critical path there is no issue but more efficient activity schedule.  If on the critical path just split the activity instead of allowing the software to make some intermittent assumption that is not visible/transparent.   

Usually it is desirable to schedule the activities as a continuous operation as intermittent work is less efficient, stopping and re-starting of an activity might mean re-mobilization of crews, the inefficiencies created might mean the duration of the segments might have to be longer than the duration of the a continuous operation.

Under the special case there are competing starts and finish constraints on an activity under interruptible scheduling it will be scheduled intermittent but without disclosing the details of how each segment is assigned.  This can be a problem when required resources are unavailable during the intermittent activity period.

There are cases when it is desirable to schedule the activity on intermittent segments because of Lag Drag.  If it makes sense to split the activity into several segments adjusted to account for the added inefficiency it should be applied only when needed and under a controlled and transparent split where you would know the scheduled start/finish, duration and float of the individual segments. At times it will be better to reduce the speed of the activity by reducing work hours or by reducing crew size.

It makes no sense to sacrifice many and perhaps hundreds of activities [A BIG DEAL] that are better scheduled as contiguous when not in the critical path.  The few that are or that eventually will be on the critical path are easily solved by simply splitting the activity or slowing the speed of activity execution [NOT A BIG DEAL]. 

At other times when there is enough project buffer it will be better to keep contiguous work.

I am not opposed to such across the board implementation if used as an easy and fail safe way to see if there is some lag drag that is critical and then use controlled individual activity splits to tackle the issue.

In Spider Project we have a float value we call Start FLEX; with Start FLEX you can know how much you can stretch the activity start.  Lag Drag is easy to identify if looking at activity float values for Start FLEX. Another way to look at the existence of Start Flex is by making it visible as a bar on the GANTT diagram.

Rafael Davila
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LAGS:

Lags and Activities are not the same thing. They complement each other but are not a substitute of each other. Time lag models a fixed elapse of time that is to be fixed in duration and that will start to elapse as soon as the predecessor happens.  It is a model intended to happen automatically.  Time lag, when used correctly makes a better and more efficient model than if using an activity as a substitute.  The model will be more efficient in terms of activity count and updating.  If not enough the need to update the substitute activities will be error prone. 

Very rarely true time lag is what shall be modeled but volume of work lag; it just happens that most software does not provide for volume of work lag and therefore many schedulers misuse time lag.

RDM highlights the importance of looking at the links table.

http://www.fplotnick.com/rdcpm/05sce-07.pdf

If you want to see the logic just take a look at the links table, it is a no-brainer.  The parameters associated with the links are so many you cannot see them at the activities table.  Links are part of the logic and part of the path; path do not happens if there are no links.  You cannot see the logic if you do not look at the links that make up the logic.  If you want to see all the links, including hard links as well as temporal resource dependencies just look for them at the links table.

Users of P3 and P6 can access the links table if using Logic League™ by Ron Winter.  You can view and manipulate your relationships like you do with activities in Primavera.

http://scheduleanalyzer.com/ll_brochure.htm

http://scheduleanalyzer.com/ll_resource.htm

If a successor must be delayed until some work is done by predecessor the lag is a volume lag and not a time lag.  That some time elapsed does not mean required predecessor work was done.  Time lag happens by mere elapsing of time while volume lag does not. If production rate slows then successor is further delayed to account for the reduced production rate.  True time lag is very rare, curing time is one such lag, most other lags are volume lag, just old software cannot model it and many schedulers do not even know the concept. Using time lag to represent volume lag is wrong.

You can use a Hammock, the model will work as expected but there will be no need to manually update lag activities.  Good to show elapsed lag, remaining lag and Relationship Slack [a measurement of the free time inside of relationships between activities].

 photo LAGD01_zpsocgl4pyw.jpg

Instead of wasting time in workarounds better use the links table.  With some formulas and a script you can disclose elapsed, remaining and slack at a single click.

 photo LAGD02_zpsd9rc5oce.jpg

Patrick Weaver
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Take out positive lags you effectively eliminate SS and FF and SF links and you have reverted to the same calculations as existed in an Activity-on-Arrow network.  

 

But......  Apart from a few situations where there should be a delay between completing an activity and starting the next (usually drying or setting time) and the lag is on a FS link the use of these other links with lags will cause calculation issues.  For an overview of the calculation problems see: http://www.mosaicprojects.com.au/PDF/Links_Lags_Ladders.pdf

Rafael Davila
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THIS ARTICLE AND THE REFERENCES ARE IN ERROR!

USACE 2015 cpm specifications - UFGS 01 32 01.00 10 from Rafael Davila