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Using lags in link between tasks vs using a separate task with duration of a lag

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Evgeny Z.
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Dear all,

May be it is a stupid question, but I could never understand the reason to use lags in links between tasks. In a simple FS link, lag would mean, that one task can only start with some delay after the first one has finished. However in reality there must be some reason for this delay (e.g. paint will dry or whatever). So in this case, why not just add an extra task with the duration of a lag? This will make the schedule more transparent also will allow more flexibility in controlling the lag as one can add also calendar, comment, give it a miningful name  etc.

So, why would someone use a lag in a link then as an opposite to adding of an extra task ?

Regards.

Evgeny

Replies

Rafael Davila
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Joined: 1 Mar 2004
Posts: 4780

Evengy,

Even if you do not split the original activity it might be that resource assignment on the predecessor activity and the activity mimicking volume lag [using dummy resources as not to mix them with resources used by other activities] will not match. It is kind of complicated that the best, or perhaps only alternative is to use the functionality as implemented in Spider.

Of course challenging and testing the functionality is good practice.

Best regards,

Rafael

Evgeny Z.
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Joined: 13 Jan 2008
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Vladimir,

I agree, that splitting tasks to emulate lags may cause problem, when doing resource leveling.

Rafael,

thought  don’t think it is related to this subject, but did definitely notice, that Spider has very small for it's functionality size of the installation package and very fast to install. Also startup as time as well as the interface and overall functionality is very fast.  This is especially if you compare it with Primavera, which took me 1 hour to install.

Regards.

Evgeny

Evgeny,

splitting activities to emulate lags may cause problems when resources are limited.

Second part of an activity may be delayed if some activity with higher priority will require resources that completed an execution of the first part.

Regards,

Vladimir

Rafael Davila
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Posts: 4780

Evgeny,

Volume Lag means that the next event [start/finish of an activity] can start only after a set-up lag volume is accomplished for the current activity event [start/finish]. The volume lag will be accomplished by the current activity [predecessor]. Lag duration can increase or decrease depending on a combination of factors, volume lag duration is not fixed as in duration lag.

Therefore in any case you will need a production type activity with required volume of work, resource loaded with corresponding resource productivity, with the same calendar as the predecessor activity [this is why when you select volume lag the calendar option is dimmed]. 

Neither MSP nor any Primavera products model volume of work, and productivity without volume of work is something I do not understand because effort does not necessarily means some volume of work was realized. Even work as defined in MSP as equivalent to effort hours makes it hard if not impossible to model same resource with different productivity assignments.

Say you have an activity for Gypsum Partitions and a successor for Paint Gypsum Partitions with a 2 days lag, 2 days after starting the Gypsum Partitions Paint Can Start, no matter if no real progress was done. With volume lag, say 300 sm of partition, only after realizing [actual or projected] the 300 sm is that Gypsum Paint can start. If this requirement is not met and you progress such occurrence it will be identified as an out-of-sequence event.

The use of volume lag as well as time lag is frequently a preferential value, same as if you start a road from B to A instead of from A to B, it does not matter if you split the activity, this does not make it pure logic as some believe.

By the way, did you noticed that downloading Spider Demo takes about 2 minutes and installing the software also takes about 2 minutes. Note that Spider Demo is a full functioning version, it takes me the same time to download and install full version and upgrades and never had any installation issue. Don't ever expect you will be given installation CD, it all takes about 20 MB.

The database architecture makes it easy to backup and deploy. It provides for live automatic backup while in use as you do not backup the whole database but individual files, full backup is as easy as the jobs are stored on separate files and you do not need to stop using the whole database for your backup similar to windows backup of your hard disk where you do not have to stop working during the backup. I am not sure about P6 but recall being required to stop using P3e [a prior version of P6] when IT people were doing a backup of the database, it was an issue [well a long coffee break]. Also if you copy a job file for e-mail exchange and it is protected the protection setup will move with it, a very convenient feature when working on a shared enviroment.

Best regards,

Rafael

Evgeny Z.
User offline. Last seen 11 weeks 3 days ago. Offline
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Gentlemen,

Thank you for your feedbacks.

I understood, that lags are better to use for things which will happen no matter what and do not require any progress monitor or control from the scheduler. Even though I must say I can't think of too many types of activities like this. Even things like drying and/or curing may be influenced by outside conditions (e.g. temperature, humidity etc) and at least would require control hence one may think of entering progress for them and potentially adjusting them.

I can also see, that tools like Powerproject and Spider offer more functionality related to links, than MSP. 

 

Rafael, Vladimir,

I think Spider volume lags can be emulated in the following ways:

For SS and SF links: splitting preceding  activity in 2 parts with correspondent durations

For FS and FF links: adding dummy task with correspondent duration.

 

Regards.

Evgeny

Rafael Davila
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Mke,

You forgot to say - links can be toggled on and off easily using the categories and that with user defined fields you can define other category sets.

Spider can do that, but how about MSP, P3, P6 and SureTrak without the help of external software, out of the box as Spider and Asta can do.

Hope you are getting better with Asta resource leveling with a few prioritization rules that work best for Asta, better than default. This will support my argument that having Asta, which I believe better than P6, is good for what we do, that Asta as well as Spider and others shall not be excluded by brand, shall be better or equal, I am sure Asta right now meets "better" requirement.

Can you post a sample job related to your issue with float, preferably in MS Project XML format. I don't believe XER is your favorite.

Some people erroneously perceive that critical path must be continuous from start [after DD] to finish, but it can be as much as a single activity. Say you have a job of 19 activities of equal duration and 17 in tandem have float except 1 on a separate path, the last activity cannot start after certain date, very late date, due to contractual requirements. In this case your critical path will consist of a single activity until float on other activities is consumed to the point of becoming part of the critical. Well the critical path is not necessarily the longest path. Calendars, constraints and resource availability can have the same effect, as well as other that while a true mathematical solution set is not understood by many who expect a clear continuous path from start to finish.

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Best regards,

Rafael

Mike,

I don't see this as a problem.

If lag finishes on non-working hours (curing may be finished on Sunday) it really means that preceding activity execution may be delayed. In any case if activities have different calendars the problem is the same - only one final activity may have zero float.

Regards,

Vladimir

Mike Testro
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Hi Rafael

I forgot to mention that in Asta you can set a category for any link and that category can be set in the legend.

Now all of you - how do you get around the fact that if a lead lag ends in a holiday period the float and critical path is affected.

The solution to this has eluded me so how do we solve it - consider this to be the "holy grail" of software usage?

Answers on one sheet of paper please.

Good luck & Best regards

Mike Testro

Rafael Davila
User offline. Last seen 2 hours 27 min ago. Offline
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Posts: 4780

Why using an activity when lag is more efficient, even less error prone as a duration lag represents a fixed wait period that will happen without interruption no matter what after it starts. Activities to mimic lag require updating while lag do not, therefore might look similar but never equal; activities will always require updating while lag will never require updating, though both might require some adjustments because schedules are dynamic, plans are dynamic.

You shall not limit the use of lag unless your software is incapable of displaying a links table. Spider and believe Asta PP display a links table that will provide all required transparency. In the links table you shall be able to see any comment that describes the lag making it transparent [or not transparent if using poor description], you shall be able to use a Lag Type Field you can use to filter and toggle on and of the link with simple formulas. If these fields are not enough then you can define as many used defined fields you can attach to the links and use for formulas, filter by any available field such as lag among, sorting etc. This principle applies to lag as well as to the SF/FS/FF link types not available in ancient CPM software but now available in almost all.

Photobucket

About lag calendar Spider provides for a fully independent assignment of lag calendar. This is not like in some [many] other software that limit you to a few rules across the board. We frequently use for curing a 24 hours calendar while predecessor/successor calendar might be different.  

CPM is about links, mere activities without links do not make a CPM model, just draw a Gantt. Software that do not provide transparent information about your links are poor. If your software does not gives you the required transparency as for you to consider inefficient work around you should consider switching.

Get into the modern functionalities.

Evgeny,

Spider Project permits to set both time and volume lags.

Volume lags cannot be modelled the way you suggest.

Besides, as Mike wrote, entering actual data you will need to enter actual duration of lag activities.

And besides if there are many lags the schedule will include a lot of dummy activities and this is not good for reports and analysis.

Regards,

Vladimir

Mike Testro
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Hi Robert

You are correct in your defintion of a Lag and the sole purpose of such links.

However an interested third pary veiwing a Lead Lag for curing would not immediately see what it was for.

A task with a title is obvious - it can even be colour coded - except P6 of course where there is only one shade of grey.

Such "Dummy" tasks merely fill in a visible gap and are therefore devoid of resources etc.

Best regards

Mike Testro

Robert Hughes
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I use lags as I believe they make the programme simpler, although that is very much a matter of opinion. A lag, such as the paint driying example given, is not an activity - there is no resource that need be applied to it and no cost, it would happen following the completion of the predecessor regardless of whether there was any resource or cost input to it. When I look at an activity I see a duration which is normally bound by resource, cost or practical/logistical constraints - a lag is not constrained in this respect and it's only input is the unavoidable, and uncontrollable elapse of time.

Mike Testro
User offline. Last seen 5 days 20 hours ago. Offline
Joined: 14 Dec 2005
Posts: 4393

Hi Evegny

You are absolutley correct in your method of using lead lags only for drying or curing periods - but make sure the lag is set to calendar days.

This is the only occasion that I will allow Lead Lags in a programme.

The disadvantage of using a dummy task linked FS is that you have to estimate the time used when setting % progress.

If you want set up a title for clarity then you can set up a buffer task if you have PowerProject or a hammock for the other software (covering a Lead Lag) and again it has to be set to 24/7.

The basic problem with any of these methods is that if the drying - curing lag ends in the middle of a holiday period then any downstream float is increased and sometimes the critical path disapears - I have not yet found a cure for this situation.

Best regards

Mike Testro