critical path

We create activities that are necessary to achieve scope goals (function and design). Float shall be calculated considering logic, time and quantity constraints.

"it is called Resource Critical Path or Critical Chain"



when you define resources as: Materials, Equipment, & Manpower, these items will compose your physical activity(s).



which means that a certain physical activity is a composition of several elements that can become critical in anyway!



may it be in time, (when calculated in terms of float / CPM), or maybe in quantity or design and function



for me, value is important and must be calculated as to represent the expected end result that the customer wants.

With the resource constraints traditional Critical Path activities may have positive floats and activities that had floats in the standard Critical Path schedule will become critical (their late performance postpones project finish).

With resource constraints true critical path is defined by all project constraints (network logic, imposed dates, calendars, resources, financing, supplies) and may consist of sctivities that are not linked with each other. It is called Resource Critical Path or Critical Chain.

Guys,



let me clarify it, Steve said "you can’t put on a roof ’til you’ve built the walls", which for me, the statement is speculative! it’s not absolute!



i never said that it is not correct, of course that’ll depend on a certain situation, say, you’re building a tunnel!



the point is, whatever the situation would be! CASH is always vital! (Critical Activities Simulation History/Histogram)!



have a nice day!

Annon,



No offence just for your info, on my experience I have done so many times a top down constructions which roof & trusses first before the walls. I hung it due to weather condition so the workers can still work during rainy days and the projects are all successful. (I know that the logic is the main discussion in here and not the physical activities.)



Regards,

Rodel

Anoon!



You said "you can’t put a roof till you’ve built the walls" I don’t agree with this one. Columns/trusses support the roof!



You never seen an Igloo?

You have taken a very simple message and rubbished it why? the message was define the logic.



As planners we must simplify complex situations so that Managers can Manager them (they are only simple people after all)



If we bamboozle people with talk of leads, lags negative float etc the message is lost.



We must rely on building a correct network on the basis of the best information to hand, and then updating it as the project changes - and it will!



Never mind the fact that the project is running late we must project when it will finish! In simple one line terms.

Steve,

I also pretty much agree with your posts though we use slightly different techniques.

I will answer your questions and hope that it will clarify unclear items.



1) "It’s not clear to me why uncertainty/variability on resource availability is any different from uncertainty/variability on an activity."



Let’s suppose that some material is supplied weekly. The quantities are known and requirements on each activity are also known - no uncertainty at all! The only uncertainty is the precedence, it is not obvious what supply activity shall precede to what consumption activity. It is not clear when and on what activity supplied material will be used. It depends on the project schedule that shall be calculated taking into account supply constraints.



2) I did not object to calculating CPM schedule for project analysis and resource optimization. But real feasible floats in the working schedule shall be calculated taking into account all project constraints. We also recommend to use NPV as project success criterion (http://www.spiderproject.ru/library/mps.ppt) but I don’t agree that adding resources is an easy task. If you can install only one crane in the construction site you shall calculate project schedule with this restriction. It is physical.



I will look at your paper later, I think that we use similar approaches.



Best Regards,

Vladimir

Steve,



"an optimized CPM schedule should be generated first" i guess everyone agree with this, but this is very hard/difficult to figure out in EPC projects, which you will need a lot of "CASH" (Critical Activities Simulation History).



"you can’t put a roof till you’ve built the walls" I don’t agree with this one. Columns/trusses support the roof!



it’s good to warm up a monday morning!

Vladimir,



As usual, I pretty much agree with everything you say -- but I feel some of these ideas are so fundamental that it is important to go deeper and discuss the implications.



First:

"4) The steel may be purchased for not one but for many activities and many times during project execution. In this case simple approach with creating link from delivery activity to consumption activity does not work. It may be impossible to know which delivery provides materials that will be used on each activity."



It’s not clear to me why uncertainty/variability on resource availability is any different from uncertainty/variability on an activity. I can think of many project situations (Which of several chemical compounds or delivery devices will prove best for a specific pharmaceutical market niche? Which platform we’re testing will be best for this system? Which precise drilling hole, and how deep, will result in an oil well?), where there is great uncertainty that will affect the start of successor activities.



Second:

"When resources are limited then classical approaches to calculating activity floats based on network logic do not work."



This does not obviate the need to generate an optimized schedule based on network logic, though. In fact, even though I suspect we are in complete agreement that a schedule that is not resource loaded and adjusted for resource unavailability must not be considered the "working" schedule (in other words, it should be the resource-limited schedule, and the working critical path should be the Resource Schedule longest path), this makes it even more important that an optimized CPM schedule be generated first:



a. An optimized schedule can often "pull" an activity’s dates to a resource’s early "window of availability" and thus avoid what would otherwise be huge delays due to later unavailability. This is especially relevant, in my experience, in multiproject organizations and with equipment resources like clean rooms, testing ranges, etc., where availability windows open and close according to the needs of other projects.



b. In the Total Project Control approach, it is absolutely crucial to identify and quantify the constraints and their effects on schedule and project profit. The optimized "classical" CPM schedule is what I’ve started calling the "Newtonian" schedule, i.e., as much as possible, the only limiting factor should be the laws of physics: you can’t proofread something till you’ve written it; you can’t put on the roof till you’ve built the walls. (If you can figure out a way around such physical laws, then see if doing it that way will increase project profit without unacceptable risk -- if it can, do it!)



c. When going from the optimized CPM schedule to the resource schedule, the delays will be due to (primarily resource) constraints. These can now be measured. In TPC, the reduction in project profit due to each constraint is called the Cost of Leveling with Unresolved Bottlenecks. And that’s the CLUB you use to spend the extra $20,000 to get the resource you need to bring in the delivery by three weeks at $50,000 in increased project profit per week. And the key is: it’s (usually?) a lot easier to change resource availability than to change the laws of physics!



d. In a continuing and multiprojected organization, it is crucial to collect data about CLUBs, by both individual resource/skill and department. The CLUB is of great value to functional managers in their neverending quest to justify staffing up to sate the needs of the projects they support. These CLUB data should be a key part of the output of the project postmortem and ABCP analysis. Failure to do this is one of the main reasons that the same resource’s unavailability delays project after project month after month and year after year, with no improvement because everyone "works around" such delays by planning activities that could be done in one day for one week, because they know the resource could take that long to become available!



For more on that, you can check out the "Post Mortem Agenda" document among the resources at:



http://www.totalprojectcontrol.com/resources/resources.html

I will repeat my remarks to Andrew’s post that were made at Critical Path Float discussion.



If resources are constrained then:

1) The longest sequence of activities may consist of activities that are not logically linked with each other (Resource Critical Path or Critical Chain).

2) Activities that belong to classical Critical Path (calculated without considering Resource, Financial and Supply constraints) may have floats (resource constrained floats) due to resource (financial, supply) limitations.

3) Resource limitations can not be represented as some activities.

4) The steel may be purchased for not one but for many activities and many times during project execution. In this case simple approach with creating link from delivery activity to consumption activity does not work. It may be impossible to know which delivery provides materials that will be used on each activity. At least before project scheduling.



When resources are limited then classical approaches to calculating activity floats based on network logic do not work.



Regards,

Vladimir

Thanks for the info, well i think it really depends on how we will link correctly our activities which will also come from a good knowledge of what we are doing specially familiarization...



Cheers,



Joseph

As long as all the logics are correct and not vague (for SF or FF links with negative links), it should not matter what portion of the total project activities fall in critical path. That is not something planner can control if that’s how the logics are. If the proportion of crticial activities at the beginning appears more, it is a management decision as to take that risk of undertaking the project. But of course, at that stage it is important to check if the calculated crical path is correct. If the management asks planner to change the programme to reduce proportion of critical activities, the simple request to make will be give you the longer time. If granted, (not likely), then you can start changing some of the SS links to FS. Anyway this is the question only at the begining of the project.



Once the project comes closer to completion, it is very likely that all the remaining activities (whether they were critical or not initially) will be critical. And project manager cannot turn away from this reality wih having 100% activities critical.

Can we be simplistic and answer the following yes or no?

1. The critical path must be the longest path from start to completion.



2. The only way activities on this path have float is because certain activies are constrained?



3. Constraints are really activities not defined on the network, (Because they may be by others outside the contractors control.) activities up to starting a steel frame are non critical because steel start is constrained by a steel delivery milestone (steel has been procured by client to a set delivery date.). All following activities are critical. The delivery milestone has behind it a series of critical activities not defined in the programme.



4. If an activity "steelwork procurement by others" had been included in the nework (from contract award to steel delivery (F-S) Then that activity would be on the critical path, and all activities on the longest path would be critical.



Hope that makes sense, from a simple building planner.

1. yes

2. yes yes

3. yes yes yes

4. yes yes yes yes

Hi all

I think that the critical path is as good as the logic used in the schedule. It depends also on the various calendars pertaining to the critical activities. All of these combined with your own definition of the critical path envelope i.e. total float =< x days may provide a less than accurate depiction of what is going to define your completion of the project. Ultimately, the schedule, particularly the critical path will reflect the thinking and experience of the project team.

I agree with what Ron Winter has to say in regards of the longest path being a more accurate tool to identify which are the activities that really drive your completion date. In fact, I go one step further. When reviewing the critical path envelope, I look at the logic driving the longest path(s) and encourage for those (and only those)critical activities to be broken down to a point where the only relationships used on the such path are FS. This practice is important when setting up the baseline. Obviously, the limitations to the above are 1) the amount of information available when building the schedule and 2) keeping the number of activities to a manageable level.

I will like to hear your thoughts on this view and approach.



Cheers



Jose

Hi Joseph,



All you need to do is to identify all activities that can be done in parallel. Also reduce your FS relationship and use SS with lag or FF with lag. By this method you can reduce the amount of critical activities.



Cheers,

Rodel

Hi all,



Just this few weeks i have tranfered to other company as a planner obscourse and they have ask me to make a programme for a small warehouse and the planning manager ask me to put the end date of the project which constrained almost all the activiries, and when i submitted the programme he saw the float on some activities are less than 5 so he told me that if the float are less than 5 i shoild consider it as critical and so i did, now the result of my programme is all critical...but they also keep on insisting that number of critical path shoul only be 20 to 30 % of the total duration...any idea or comment regarding this method guys?



Best regards,



Joseph

Hi all,



My perception of critical % is depending on the risk that your company want to take when they bid and won the project. First is the entire deliverable milestone that required to achieve then the activity that driving it. Those things are already determined before bidding the project based on risk management analysis and constructability of the project. If a project is pre-determined risk free then ( I don’t know if there is one) the critical % is determined as the project go on which has been discussed and agreed based on the construction methodology and the risk against its activity which draw a critical activity to achieve the deliverable milestone to completion.



An EPCM project has a higher critical % because the critical activities are being driven starting from the design then followed by the procurement and last is construction and commissioning. But all of these are already determined from the bidding stages as the deliverable milestones are already set.



If you can follow the risk assessment % and patterned your critical % of your project based on the agreed construction methodology then that would be a good start. The critical % will change as the project go on depending on the achievement of deliverable milestone. A new logic or sequencing maybe implemented based on the new agreed construction methodology if the risk and critical % are getting higher to reduced the critically of the project to attained the deliverable milestone to date.



Any changes on the design/ change of scope and variation may drive the criticality of your project. It may reduce or higher depending on your new scopes where your deliverable milestone is also changed.



Thus it makes sense?



Regards,

Rodel

hi guys....



remember that the critical path will depend on the logic that you would create on the activities............



there is no required % of activities in a programme for a critical path..........



you have the full control on how you would create the programme......



be logical on your links dont overdo your links

hi all,

based on my understanding, critical path must have the nearest picture should be and will be as at site. The main thing you should know what the cause of the critical how to planned /solve for critical from running smoothly without obstruct. once one of the critical activities delay it will impact the related link and may cause 2 activities and so on.... then you get the great impact to control more activities which may cause overlook.

just an idea to look about critical.... to be aware when one critical activities stop running.



regards

So Tom then you also say, That the planner should now, whats normal in his schedule towards critical activities. And when the output shows something else, he should look into his schedule to see what’s right (or not)

So dont just relay on the output of your schedule but try to look at the output from the users (builders,managers, etc) point of view. You cannot give one answer because it highly debends on what you’re planning.



Kind Regards



Henk

The idea of having some optimal proportion of activities on or near the critical path is a valid concept in my opinion, but I don’t think there can be a general rule regarding what that proportion should be without considering the scope of the project.



During the initiation of a large, complex project, we will typically focus our attention on the near-critical activities, and with sufficient granularity (thanks Stephen) to achieve the primary purpose of the exercise - i.e. confidently predicting and committing to the project’s schedule milestones. If this initial planning succeeds (i.e. the project is approved for further development or we win the contract), then we will proceed to develop the baseline schedule, in particular fleshing out the non-critical activities (i.e. increasing their granularity) to the extent that the schedule becomes a useful management tool (especially important if earned value reporting is to be done). Thus, the overall proportion of activities that are at or near the critical path should decrease as the schedule becomes "complete". Conversely, a schedule with a higher-than-specified proportion of critical-path activities might be presumed to be "incomplete". While I recognize the concept, I’ve never actually applied it and would be a little dubious if faced with such a requirement. Simple numerical targets are an invitation to playing games....



So, to address Eduardo’s original question,

what’s a suitable percentage of critical/near-critical activities - I don’t know, but I’m sure it depends on the scope and complexity of the project. For a (horizontal) chemical plant project with many opportunities for parallel work, then the percentage should be lower. For a large civil construction project where parallel work is not as possible (high-rise building, dam), then the percentage should be higher. The best reference is the acceptable baseline schedule from the previous similar projects.

Best of Luck, tom

Hi,



Your critical path will depend on how you link your activity to each other, and so your critical path will depend on it.



Mabuhay,



joseph

I (think) I agree Henk. Surely no-one has the time to put EVERY piece of logic into EVERY programme they produce ?? A prime example being at tender stage, where aside from the 1/5 risk of not actually winning the job, information, phasing & construction could substantially change before the job starts on site. I realise I’m opening myself up to professional criticism but this is the reality for nearly every programme I’ve ever seen (whether by me or others).



An experienced planner will know which pieces of logic are important, which are on the fringes of criticality, and which will be more or less redundant. The software should merely confirm the planners gut feeling of critical path, and rarely dictate it. It is also important to remember that a critical path can drastically change just from the small alteration of an activity duration, lead / lag or introduction of a single logic link.



I see this time and again when progressing projects. Progress durations can be highly subjective, and the difference of a few days progress for just a couple of activities will alter future criticality in a big way. (The bonus of this being that without too much manipulation, a progress programme can be made to tell the story that you want it to tell - Henk’s "being creative with the software" idea ??).



Right, those admissions should ensure I am hung, drawn & quartered by the A-nal planning brigade !



Nothing in Planning is ever black /white, right / wrong, just shades of different ideas and accuracy.

I think the point is what do you want people to read out of your planning and critical path.

One thing is for sure. When the manager asks you what the critical path is he dont want to see 50% of the schedule.



So i think a good planning shows a critical path that gives you an idee what are the most important activities that are responsible for the project duration.

When there are to many activities you better start being creative with the software (works for me)



Kind regards

Henk

The idea of "limiting" the number of CP activities seems to me as strange as Emperor Joseph of Austria’s complaint that Mozart’s latest opus had "too many notes." Mozart’s reply was that it had just the right number, no more or less.



I actually can understand wanting MORE activities on the CP, as that would suggest greater breakdown and granularity, and granularity is a risk reduction technique.



I do believe that it’s vital to know HOW critical each CP activity is, by which I mean how much time it’s adding to the project duration (i.e., its DRAG). For those who are interested, I explore DRAG in the fourth ("Delay Tactics") and fifth articles ("The Whips and Scorns of Time," due out Nov 30th) in the ongoing series on Total Project Control at [email protected]:



http://www.projectsatwork.com/departments/methods-means/

I have seen many scheduling specifications that require the Baseline Schedule to have no more than 30% critical activities and no more than 50% near-critical. I have never seen a specification detailing any percentages of this type for schedule updates. Good luck!

And more ...



Depending on the characteristics of the activity, one could be more critical for the project than if u have many.



Regards.

Charleston is right - could be 1 or could be all of them!

Hi,



Depends on the project requirements.



There is no clear acceptable theory or law to govern the number of critical activities.



cheers,



charlie