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Allow Long Lag or Use Soft Constraints - Question

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Robert Dawkins
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I represent a client that specifically requires limited use of lag if used at all. I have a situation where the contractor chose to delay a series of activates to improve the schedule. The schedule includes several buildings each could be considered a “phase” but the terminology is not in the contract.  When I analyzed the schedule I found that to move these activities the contractor used a 150 day lag in a FS predecessor relationship. Their reasoning behind this was that there were no other logical predeccessors to tie to. This occurred in 4 different areas/buildings. So I have to make a decision.

Allow the lag? I don’t like this because it is hidden in the relationship and the contract specifically states lags must be reasonable and not used in place of realistic original durations, must not be in place to artificially absorb float, or to replace proper schedule logic.

Soft constrain the activities (Start on or After) Leaning toward this even though I hate constraints. This is allowed in the specifications only if I approve it. This was talked about in our last scheduling meeting and my response was to remove the Lag, use the soft constraints and add comments in the note tab. At least constraints are visible (*).

Another option I was considering was having them create a descriptive start milestone in the general WBS section and apply the constraint to it and then drive the other activity logic using SS tie.

 

I would appreciate any suggestions or comments on this.

Replies

Patrick Weaver
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There is no lag and there is no 'activity' - a lag represents a necessary delay between two activities, the situation described is not a necessary dealy.  Lags also affect float on the path, the 150-day lag removes 150 days of float on that path and may well end up distorting the schedule. 

If there is an overt management decision to delay work until after a particular time the best way to show this is by a soft constraint but put it on a milestone. Describe the milestone accurately (eg, contractor delayed start) and put the soft constraint on the milestone. Assemble the rest of the logic normally. 

The advantage of this is if the contract decides later to adjust its management decision the milestone can be removed or altered but the underlying network logic, critical paths, etc all remain. 

Zoltan Palffy
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does not hurt to ask or to get a second opnion 

Robert Dawkins
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Thanks. The worse thing I do is secound guess myself. I just want thing to be right.

Robert Dawkins
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Joined: 16 Feb 2012
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Thanks. The worse thing I do is secound guess myself. I just want thing to be right.

Rafael Davila
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Different to activities Lag is intended to be taken care automatically.

Once an activity starts the duration of the in progress lag starts to elapse. Then when updating the software automatically distributes total lag as elapsed lag and remaining lag keeping the successor in place along the updates.  If it is necessary to change the successor start date different to initial target date, then total lag might need to be adjusted as to adjust remaining lag and therefore start date of successor activity.

Some have proposed for software developers to include better ways to control remaining lag once lag stats to elapse but still active.

Making CPM More Transparent

In-Progress Lag Report and Value: Remaining Lag should be displayed just as remaining duration is shown and editable. The CPM feature of Remaining Duration was added so that Schedulers could monitor and change this calculated duration result. Lags are to relationships what durations are to activities. Why should Remaining Lag calculations be discarded by the software instead of saved and displayed? Why should we be unable to indicate that actual remaining lag is only 2 days instead of the calculated 4 days? Part of the reason CPM Schedules are so hard to understand is that fact that not all of the data used is displayed.

https://mosaicprojects.com.au/PDF/Links_Lags_Ladders.pdf

Lags should not be used simply to create a space between two activities ‘for convenience’ or to make the schedule look correct. These ‘leaps of logic’ bypass true network logic by linking tasks with inherent gaps in time between the activities and can be misleading and may cause computational errors when used; the effect is similar to putting artificial constraints in the schedule and should be discouraged.  It is not uncommon to limit date constraints to contractual constraints that are external to schedule logic. 

Best practice is to use true network logic.  My recommendation to use date constraints as you suggest is based on the assumption the constraint is external and not based on schedule logic.  All such date Constraints shall be justified unless they represent contractual milestones.  My assumption is that such constraints are duly justified and do not represent in any way a bypass of true logic. 

In some cases true network logic is born from resource leveling that creates temporal resouce dependencies.  In some cases use of spatial resources is best approach as manual leveling using artificial logic is not a good idea.  Artificial logic should be of last resort, shall be disclosed and its use justified. 

http://www.pmknowledgecenter.com/node/104

Spatial resources are required by a group of activities, rather than a single activity as renewable resources. The spatial resource is occupied from the first moment an activity from the group starts until the finish of all activities from that group. 

Zoltan Palffy
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When using a long lag such as that it ineffectivly is hidden and can not really be seen unless you go into the sofware.

As Rafael has mnetioned a long lag such as this would require updating every reporting period where by you woul have to reduce the duration of the lag based on the update interval. 

A better solution would be to make the lag an activity this way it would be visibale and you can give a description to the activity which you cannot do with a lag. You can name the activity something like access to next phase or next sequence.

This way everyone can see it and it is in plan sight and is understandable and supports the reasoning of being a predecessor to the start of the next phase.

As Rafael also mentioned using a milestone with a start on or after constraint is also another way to do this. However if the goal is not to have a constraint then add the activity 

 

Rafael Davila
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Using an activity to represent lag might be another option. 

  • Pro: visibility when software cannot disclose the relationships table.
  • Con: lag activity requires updating while elapsed lag calculations are automatic. 

There are no fixed rules, at times it is better to use time lag, at other times volume lag different to time lag is a better choice, at other times using an activity to represent lag is a better choice, at other times using a soft constraint such as start no earlier than is a better choice.  In this particular case the use of soft constraint as you suggest seems like the best option. 

Using a start milestone for each phase makes sense when several activities within the phase can start at the same time.  I see no difference between a SS and FS successor to a milestone as for a milestone S=F.  

The software I use, Spider Project, allows for multiple WBS Structures so I would keep the milestone close to the activities it drives under one WBS structure and keep a separate WBS Structure that meets your preferences. 

Good luck.