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Critical Activities - Rule of thumb

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Sukumaran Subaram...
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When we develop a schedule for a project with total number of activities amounting 6,000 (example), what is the allowable criteria for the critical activities.

Is it 5% to 10% of total number of activities or we shall let the longest path to control the critical activities.

Your advise is appreciated.


Regards.

Replies

Stephen Devaux
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Man, how come nuhbody doan nevah tek muh serious?

Fraternally in project management,

Steve the Bajan

Anning Sofi
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Sukumaran, take heed and listen to Steve's (Stephen D) posting below, so you can properly advise your client. Seriously.

Rafael Davila
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Great rule of dumb.

Mike Testro
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Hi Steve

I am delighted to say that I have never read such unadulterated claptrap as your last thread.

Thanks for the laugh.

Best regards

Mike Testro

Stephen Devaux
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The correct number of critical path activities in a project varies a little according to the type of project. In a pharmaceutical development project, for instance, the potential impact of poor quality on patients' lives means that not more than 6.67% of activities should be on the critical path. (NOTE: this does NOT mean 6.66667% -- it means EXACTLY 6.67000%!).   In nuclear power outages, it depends on the type of plant: boiling water plant outages should have 7.74% of the activities on the critical path, while pressurized water plants should be at 8.41%. 

In a construction project, 5.93% of activities should normally be on the critical path. However, this can change depending on the climate where the work is being done:

  • For every three days that the humidity rises to more than 84%, the number of critical path activities MUST be reduced by three activities, and
  • For every five days that the humidity never rises above 50% AND the average cloud cover is greater than 35.7%, the number of critical path activities MUST be increased by four. 

This can sometimes cause slight difficulties: some work may have to be added if there are a lot of cloudy days with low humidity, and this can result in law suits to determine whether the customer or the contractor should pay for the additional work. (In the most recent case in Tortola, the court ruled that the most prominent meteorologist located within a fifty kilometer radius should pay for the extra work, a decision that is, however, being appealed.)

Perhaps the most knotty problem that can be caused by a climate-driven change in the required number of critical path activities is when a roof has to be omitted from a building in order to reduce the number of CP activities.  As a result, it is often a good idea to plan for an additional and separate roof on an outbuilding, such as a garage. Thus when required, the garage roof could be left out (although it obviously depends on circumstances -- if there are severe budgetary constraints, it would cut costs more to complete the garage roof and omit the roof for the main building. This is where the art of project management comes in!)

In software projects utilizing agile methodologies, it is regarded as mandatory that not less than 105% of activities (to allow for re-work) be on the critical path.

Hope this helps.

Fraternally in project mangement,

Steve the Bajan

Anning Sofi
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What is this 5% or 10% number of activities of the total activities as criteria for critical path?

The number of activities has nothing to do with the definition of critical path.  Look up the meaning of P6 and understand it.

Also, In P6, it is either the Longest Path or  path of those activities whose total Float is equal to less than 0 (depends on your setting). There is no option that says critical path = 5% or 10%

 

Anoon Iimos
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just want to highlight what Dan wrote as follows:

 

"Whichever Schedule Network Analysis Technique you are going to use, you must work off of the Project Scope Statement,as approved by your Sponsor,Owner ,in the Project Charter or Contract. My opinion is to make sure the Sope Statement(s) are written with the utmost care,with input from your Key Stakeholders and that the Whole Project Team has committed,'Buy In' is the Most Critical of Any Path to Successful Project Completion".

dan tab
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The Critical Path(s) ,and there can be moe than one,in addition too multiple near-critical paths are determined without adding resources. Critical Paths are the path thru the Schedule Network that are determined to have Zero Float,which makes the Activities populating the schedule ,Crtical. No Activity on the CP can be delayed without adding Duration to the Scheduled Activities,thus overrunning completion date or Critical Milestones in the Scedule. I have been somewhat supprised by the lack of mention that their are multiple types of schedules, especially the Baseline Schedule,which is used ,once completed,for All Parties to the Contract to Agree to. Critical Path can be comprised in many ways,I have seen it used in a waterfall and rolling wave type planning method,mostly in 'fast tracking' projects,with an artificial end date,(known to all as hypothetical) well within known Actual Completion Date, and this is obviously done when All is not known about the Project,but the ACD.. This allows for Resources to be loaded and activities to begin. After the CP has been developed,Resources are added,creating a Critical Chain, buffers can be added to near critical paths at the begining and end to keep the CP from slipping. The Resource Constraind Critical Path is known as the 'Critical Chain' The Amount of 'buffer' allows for uncertainty ,Risks, to completion end dates of either Target Milestones,or known Actual Finish Dates. This 'practice has been advocated by PMI for over 20 yrs.,but has not really caught on. Whichever Schedule Network Analysis Technique you are going to use, you must work off of the Project Scope Statement,as approved by your Sponsor,Owner ,in the Project Charter or Contract. My opinion is to make sure the Sope Statement(s) are written with the utmost care,with input from your Key Stakeholders and that the Whole Project Team has committed,'Buy In' is the Most Critical of Any Path to Successful Project Completion. I realize that I have not captured all of the considerations in 'project scheduling',and I do not consider myself to be a 'master' at scheduling,I'm just merely trying to elicitate more conversation on the subject.
Anoon Iimos
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Any Project cannot be done without resources, so I supposed for now, "Floats" and "Critical Path" derived from schedules or programs that were not resource loaded are meaningless (obsolete).

"Resource Critical Path" and "Resource Floats" - would be the future!

Patrick,

I don't agree. There are resource constrained floats, and early and late dates in resource constrained schedules. Resource constrained float shows what activity execution delay will not postpone project finish in current resource constrained schedule. There is also Resource Critical Path, etc.

For more information look at publications on www.spiderproject.com or/and try Spider Project Demo.

Best Regards,

Vladimir

 

Patrick Weaver
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The use of artificial constraints or unnecessary logic to create or reduce the number of critical activities in a Time Analysed schedule is to be avoided.

However, a good heuristic is once the schedule has more then 20% of critical activities the probability of it completing within the planned duration is very low.

As soon as you resource schedule/level there can be no concept of float. Move any activity and you need to reschedule to re-balance resource demands.

There is a collection of papers on our scheduling page at http://www.mosaicprojects.com.au/Planning.html#Roles that discuss good scheduling practice.

Rafael Davila
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The requirement for a schedule to have a limit on the amount of critical activities is one of the most stupid requirements I ever heard, perhaps a requirement by wannabe gurus that are just keyboard jockeys.

A 4 activities schedule all in tandem will have 100% of its activities critical but you can add as many irrelevant activities to reduce the percentage to less than 1% and still have embedded float reduction tricks, is a ridiculous requirement.

The requirement shall be that the reviewer knows what he is reviewing, then he will be able to confirm if the plan is reasonable and has no float reduction embedded, forget about those stupid requirements.

The most common float reduction technique is to increase non critical activities duration, this can be verified if you require the schedule to be cost and resource loaded with physical quantities tied to the activities, if the reviewer know how to build it and how to estimate the job he will be able to find out most duration inflated activities. A limited duration inflation will make it a bit fair as taking away all float from the contractor and making it all available to the Owner shifts too much the risk on favor of the Owner.

Daniel Limson
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Hi Lizz,

Interesting questions! Is this a quizz? Anyway, since i am not busy at the moment, i might as well answer your questions.

We normally do this exercise "resource scheduling" to optimize usage of resources and of course this will translate to $$$ at the ned of the day. As you know, in construction everything is dependent on resources (cash, labour, materials & equipment) to complete a task and if you are on the contractor's side then it is in your best interest to perform such excercise.  If you have limited resources then the programme will become resource driven and this will (of course) affect overall progress.

Your 2nd question, First of all you need to define what you consider critical, it may range from 14 days to 0 days. Normally activities becomes critical because they are tied to a key date, completion date or an interface date defined in the contract. Contract key dates normally have penalties attached with them in the  form of liquidated damages or general damages (that is if you fail to meet this important key dates).

An activity or a series of activities can become critical when the total float is less than what you consider critical.

 

Best regards,

Daniel

 

lizz lizz
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hi guys!

i've some questions here,

1) How resource schedulling affect overall project planning and progress.

2) Why some related work activities in a construction works programme become critical to the project.

Any opinion.

Regards

Lizz

Stephen Devaux
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Raj,

Thanks for the clarification. I understand now.
Raj Maurya
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Stephen, I agree with you that descretionary dependencies should be added after the schedule has been assembled to make the schedule complete.Sometimes some management decision(eg financial investment decision) required to proceed with the current work and the decision may be dependent on other activity not related with your current work. Then it is required to put soft logic with the decision piont or with the other works progress.This was my understanding of soft logic as well as you said descretionary dependencies.
thanks
Philip Jonker
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Hi Guys,

I think Bernard hit the nail on the head, plan the project accordng the the standard norms, and look at the end date, should it be later than the expected date, go to the critical path/s and check where things can be accelerated, by adding extra resources, taking into consideration constraints such as space, access, etc, and then look at increasing working hours, and then look at the extra supervision, and things like lighting, etc the have to be put into place when working shifts. Of course, your initial schedule may well be within the time frame, then you have the obvious benefit of being able to resource schedule, and try and make savings, by optimising resources.

The point is you must be able to judge whether the allocated time for the project is adequate, and this requires good practical scheduling. There is always a way of solving a problem, as long as you have defined what the problem is.

The ’rule of thumb’ is therefore not a point. For those that do not know the origin of the rule of thumb, it is used in electricity, to determine the magnetic poles of a coil/electro magnet, according to the direction of the current. Similar uses are used in steel wire ropes, to determine the lay of the ropes.

What this means is the rule of thumb is a practical method of determining things, therefore, be practical in your planning.

Regards,

Philip
Stephen Devaux
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"Then you need some soft logic as well as hard logic to develop the schedule."

Raj, I’m not quite clear exactly what you mean by "soft logic." If what you mean is what the PMBOK Guide calls "soft" or "discretionary" dependencies (those not mandated by the logic of the work), I teach that no discretionary dependencies should be included in the schedule until AFTER the schedule has been assembled initially without them. The reason is so as to be able to determine the DRAG and DRAG Cost of each and all discretionary constraints, if they impact the critical path.
Raj Maurya
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If you are planning for client and your project is very big some thing like field development with so many facilities and studies. Then you need some soft logic as well as hard logic to develop the schedule. There may be the case you would not be able to say yes this is the exact critical path. Then you will have to do some assumptions and identify the critical activities that could be on critical path. I think in this case you can think about the optimum number of critical activities and make the plan to put on the table of board or management meeting.
Eric Chou
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I always joke about phenomena in the planning profession. There are two kinds of schedulers. One is the guy creating a schedule, the other one is the guy reviewing the schedule. However, there is no one using the schedule. The one reviewing the schedule always think the other is to fool him and the schedule is not good. On the other hand, the one creating the schedule always needs more time to finish the schedule to satisfy the reviewers instead of the users. I spent most of my career creating schedules.

I think that you guys just get things way too complicated. I don’t think any rule of thumbs can replace your knowledge about the scope of the work you are planning. A professional scheduler is like a superintendent with a specialty on analyzing the scheduling risk. For example, if you work on a building project, you have to know the scope of work and plan a realistic schedule. One the other hand, if you are to review the submitted schedule from contractor, you should know even more about the scope and contractor’s plan. In my opinion, the statistics of the data from a schedule serves no relevance to the feasibility of a schedule. A schedule can have a “reasonable’ data, however, it can reflect no realistic planning and wrong critical path.

However, I think that some percentage requirements for the critical path in the spec are used to discourage the contractor from submitting a claim oriented schedule. Normally, those percentages are set at a very conservative level such as 30%. If a schedule really has high percentage numbers of activities on the critical path, it means the contractor is using too many “soft logics” in the schedule, which should be taken out or carefully justified. In this case, the scheduler’s knowledge about the scope is even more important. At the same time, this will serve as a two edge sword too. Contractor’s delay will have the same impacts to the schedule also. Honestly, I don’t think most contractors will try to do that based on my experiences. As you know, contractors definitely will carry more schedule risk than owners.

Note: Soft logics are logics connected by preference, not inherent logics such as hard logics.

Eric Chou, PE
HTC Project Controls, Inc.
www.HTCProjectControls.com
Rajesh Moolakkandam
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One can know the approximate minimum number of critical activities in a base schedule.But this is not a thunb rule. Suppose the project duration is 400 days & the maximum duration of an activity is decided as 20 days, then he/she will have minimum 20 (400/20) actvities in critical path.It also depends on how you are defining a critical activity ie: as longest path or based on the foat duration.Also the number of activities on a critical path will change as you update the schedule.One ritial activity in the current update may not be critical in the next update.
Bernard Ertl
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Hi Stephen, by target date, I meant the (initial) date the critical path ended. But either way, your comment is indicative of what I meant by management/owner expectation/acceptance.

Bernard Ertl
eTaskMaker Project Planning Software
Stephen Devaux
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"The larger the percentage of the project that is critical or near critical, the greater the risk will be for overrunning the target completion date IMO."

Bernard, this is true, unless the schedule got "tight" because the float was been squeezed out by the schedule optimization process, thus leaving a greater schedule reserve at the end. In other words, if I have a contract delivery date of July 31 and a big delay/acceleration penalty/incentive, I’d probably rather have a tight working schedule that concludes July 1 with 30 days of schedule reserve than a "loose" working schedule that concludes July 20 with 11 days of schedule reserve.

I know you were not saying otherwise. I mention this only because, while I fully agree with your point, I’ve also seen it used in some cases as a reason for not doing adequate optimization.
Bernard Ertl
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The larger the percentage of the project that is critical or near critical, the greater the risk will be for overrunning the target completion date IMO.

The percentage of criticality can be considered a measure of tightness or flexibility in the schedule. The tighter the schedule (higher the percentage of critical/near critical work), the greater the chance that a delay/problem could impact the end date.

The optimal percentage of critical/near critical work is going to be a function of many variables including the management’s/owner’s acceptable risk threshold, quality supervision availability and the nature of the project.

Bernard Ertl
eTaskMaker Project Planning Software
Sukumaran Subaram...
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Hi guys,

TQ for the advise.

Since all the planners have a different view, do we need to standardize the requirement i.e. set a guideline?

It will be a great helpful when we develop a new schedule and to advise the client.

Any suggestion.

Regards.
Kashyap Mothali
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In addition to what’s been said, you might also want to check the Specifications. I have come across certain Govt. Specs which dictate the permissible number (or percentage) of critical activities on a project.
Ronald Winter
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Actually, I have begun to see a ’rule of thumb’ being used in many specifications recently. I have see the requirement for 30% or less on the critical path and 50% or less on the near-critical path (10 days of float or fewer.)

Hope that this helps. Good luck!
Philip Jonker
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Naand Jaco, hi guys,

I support Jaco, but he meant to say good planners. The percentage of critical actities is dependant on the the project, not the planner or any other factor. There is no real rule of thumb, if the project is a cross-continental pipeline, there might be only one CP (I am learning the jargon:-) )

If it is a fast track plant or a outage, the may be twenty CP’s, it is dependant on the situation, so stop trying to weigh or put percentages to everything, think for yourself.

Regards

Philip
Jaco Stadler
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No Rule of Thumb.

If you take two planners and give them the same thing "schedule" to do you will find a different amount of near critical activity’s. You might even find two different CP. I will suggest you have a look at CPM.
Ernesto Montales
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Good day!

Critical activies is defined by the following:

1. The longest path on the network.
2. The last activities to completed.

I have not really encountered a rule of thumb on how much in terms of quantity the critical activies in a program. It depends on several factor the assumptions, methology of construction, site condition,whether your programme is time driven or resource driven and depends on what your client requires.

Hope this sched some light on the subject matter.

Regards

E. Montales