Number of cirtical Activity

I had a wonderful time trying to explain the difference between the Critical Path(s) and Longest Path to one of my PMs...



What helped was fiddling with the ’must finish by’ date on P3e when I was talking him through it. Started off with it on the last day of the job (this was a tender prog, so no progress was shown), that showed him the longest path and (in this instance) the only critical path for this finish date. I then brought in the finish by date a day at a time, each time scheduling and showing him all of his ’new critical activities’ and what I referred to as ’minor critical paths’ (i.e. paths with zero or neg float, but with more float than the longest path). By George he got it.... eventually

Hi,



Is there any thumb rule in a CPM schedule, approximatly how many critical activity and maxium value for float should be?



To answer you directly, critical activities should be at most 25% of the total activities. The lesser percentage is better as this mean you have more floats or buffer. With regards to the maximum value of float, this will depend on your length of contract but as long as you follow strictly the correct relationship of the activities, it will give you reasonable float but you must give more concern on the minimum float wherein most cases, we use that zero float is critical but I would like to advice you that if your project will last 3-5 years you must have atleast a minimum float of 30 days and for short duration at least you have 2 weeks. Because mostly of all projects will be delayed and during updating, if the owner/consultants and your management sees negative floats, they will surely make your days like hell of questioning your programme and preparing a lot of recovery programmes and many more.....



best regards

Actually longest path (as implemented by Primavera) and float values are mostly independent of each other. The longest path (when implemented properly) is ALWAYS the critical path, while activities with zero total float are not always on the critical path. We have multiple activity calendars, interruptible activities, float calculation method, and constraints to thank for this.



You might want to refer to my Published Articles page, http://www.ronwinterconsulting.com/published.htm to view my background information on Longest Path (“Longest Path Value”) and how to use longest path to measure the critical paths to different contractual milestones in the same schedule (“Managing Multiple Milestones.”) Good luck!

I agree Ronald. The ’Longest Path’ is also much easier for Project managers and site teams to understand. However,the planner still needs to keep an eye on any secondary paths that are near critical and relay that information to the team.



Cheers,

Another quick answer is to forget looking at float to determine the ’critival path’ and look to the longest path function instead. The longest path is the true definition of the ’critical path.’ Good luck!

Hi.

Keeping it simple is always good advice; however, don’t forget the schedule is a control tool. So the level of detail of a schedule must be determined by the level of detail one is willing to control progress at, which is also tied to resources and budget available for control.

Is anybody against this advise?

The model shall be simple to manage, easy to understand, and supply project stakeholders with reliable information. I think that nobody is against these rules.

Clive,



Keep programmes simple and manageable, really good advice to anybody.



Chris Oggham

As far as I’ve read, and that includes O´Brien´s books (He was in the original MIT team that developed CPM), there is no such one critical path limit.



It all depends on network logic and activity durations. You could even end up with a project that´s all critical, regarless of how many branches or sub-chains the network includes.



What computers have made easy is taking the schedule beyond the pure cpm procedure, to include constrains, leveling, etc., which affect the original critical path concept.



For example, Primavera lets you decide whether or not to show open ended activities as critical. Open ended activities make it impossible to compute a true CPM critical path (or several).

I think it’s possible to have multiple critical path..

Hi Alex,



I agree with you, there is no difference. Ideally there should only be one critical path, however constraints such as imposed Dates, a customer driven work package or other factors can give you multiple critical paths. This will apply whether you are using a piece of paper or a computer.



I also agree with your view of the computer as a tool to help you with the identification of critical activities. It does make things easier, but the principles are identical whether you use a computer or a piece of paper and a pencil.



Regards



Chris Oggham

Hello Alex,



The general principle of PERT/CPM was that there is only one critical path. If you will read the books about PERT/CPM in the early 1970 until late 1980 (before the advent of Primavera), you will be come to know how PERT/CPM was done. It was really very hard to update the schedule.



I saw it done in my early teens when my father was the project manager in heavy civil/infrastructure project, I did it when i studied civil engineering (particualr subject was about construction management)and I did it in my early years after graduation from university. The exercise was really a pain in the ###.



The new generation of planners are lucky, they have not been through what was done before. They are like the planning mascot, innocence planning mindset and computer suavy (enjoying the banging of finger in the keyboard).



Cheers,



Charlie

Charlie



Your second part of the posting is a bit ...

I am not sure there will be a different if you are planning in a computer software or on a piece of paper. The think that you mention in a software you need a client milestone..... it is the same if you are using a piece of paper is it??



My view is computer is only a tool, the user is the key to unlock its power. It helps us to identify critical path that may not be possible identify in a piece of paper. That is why you may have a multipule Critical Path.



Alex

Hello Kanan,



In principle (also in the good ole days of PERT/CPM) there is only one critical path. The number of critical activities were, as much as possible, minimal.



In my experience this happened because I did it manually in a cross section paper the size of A0 sheet. It was so tedious and time consumming to go to to much level of details.



In the advent of computer software for project management, Primavera P3 ver 3.1, the number of critical path could be more than one. This is specially true when you contraint the client milestone into mandatory constraint. As a consequence the number of critical activities are quite a lot during the preparation of baseline schedule. As you update the baseline schedule, it could be possible that the number of critical activities may increase.



There are no general rules in the number of critical activity.



Cheers,



Charlie

Hi!

There is no thumb rule. It depends on the no. of activities in the schedule and the total duration of the project.But yes, if the float value is very large;it’s an indication that relationships have not been assigned properly.

Regards,

Suraj