Carlos,

Take a look at the following discussion and try the simple 2 activities sample job for yourself.

http://www.planningplanet.com/forum/forum_post.asp?fid=1&Cat=7&Top=67302

Unfortunately Andy is correct in his assessment, but it does not means you cannot do it right. As a matter of fact I believe most exception to the rule are PP members.

About how an algorithmic critical path is defined, this is simple for CPM without resource constraining, with resource constraining you start separating kids from grownups.

For a simple algorithmic procedure for the traditional CPM computations without resource leveling follow the link:

http://www.cob.sjsu.edu/facstaff/davis_r/courses/QBAreader/cpmalgorithm....

I still believe you will be better off by means of a sample manual computation found in any introductory CPM book.

An algorithm for resource crtical path and float is more complicated and will deppend on the particular resource constraing algorithm used. I believe the particular resource leveling algorithm will yield the results. I still do not understand why most software vendors have not figured it out. Not even Microsoft, perhaps Bill Gates is not as smart as the media makes us believe he is, why his team is not capable?

For resource leveling algorithm you might select the traditional algorithm that in most software is far from optimal, in Spider Project you have other advanced algorithms that let you have some control on your prioritization up to an optimal or near optimal solution without having to trick the software, this will consistently yield excellent results. Remember none of the major software available with the exception of Spider Project will give you true critical path and true float under limited resource constraining.

Best regards,
Rafael

a quote from Andy’s

"most of all, can the rest of the project team understand it?"

MOST OF ALL, CAN THE REST OF THE PROJECT TEAM UNDERSTAND IT? should have been written all in capital letters, without it you got nothing.

Ironically how many schedules I have seen that are far from understandable. At times too much detail gets you lost and make the team to lose interest on the schedule.

Rafael, very succinct post - I love it. However, I also agree with you Shahul in that while baselines & longest path should be the same as long as the baseline logic and the schedule is ’built’ correctly, then your statement holds true.


But, unfortunately, how many of us examine baseline schedules to find that there is some really crazy logic in there that we want to change but cannot touch because the baseline has already been approved! All we can do is put our own ’repairing’ logic. not pretty, but can often work out OK.

I think it is encumbent on all of us to keep monitoring both the crtical and the longest paths, even if we don’t issue them, to keep a check on what the logic is doing as a result of our logic ’repairs’

However, the bottom line is one of practibility of the schedule. Does it make sense, is it workable and most of all, can the rest of the project team understand it? To me this is the most important 3 points of all.

Whaddya all reckon?

Your welcome Shahul, keep challenging everyone, I always follow your postings and always something good I learn from these.

Shahul,

From the following reference you will find that longest path as defined by P3, and maybe by P6 is kind of shortsighted. We might not be talking about the same thing, I was referring to Ron Winter’s definition.

http://scheduleanalyzer.com/sa_long_theory.htm

Longest Path before LONGEST PATH Software
Primavera Systems, Inc., the makers of Primavera Project Planner (P3) scheduling software defines ‘Longest Path’ as the string of directly related activities that comprise the longest path from the data date to the last activity in the schedule. This definition does not concern itself with “float.” It includes activities that might otherwise be left out by the standard definition of Critical Path.

This only solves the issue for when critical path activities use different calendars.

Best regards,
Rafael

Andy,

Yes, critical path should always be shown, but true critical path and not the original defintition for critical path when CPM was in its infancy, with only SF and no lag, no resources consideration. When it started to evolve into a more advanced modeling technique the computation of float was never adjusted as to keep its true meaning under these advanced features.

With regard to constrained dates, in particular constrained finish dates that force the software to redefine the backward pass using dates other than the logical dates are another issue that confuses especially when several constrained dates converge in the backward pass.

The idea of fooling the software to compute backward pass using constraints the redefine float in unexpected ways is an aberration proposed long ago by someone to please Owners incapable of reading true CPM computations, this to create negative lag results when a particular path is behind schedule, perceived as a good idea to warn about the project being behind schedule.

In order to eliminate the chaos this creates some schedulers resort to temporarily eliminate these constraints as to understand the schedule, then they insert back these artificial constraints to please the Owner desire to see a lot of negative lag warnings. But this is time consuming, can induce errors in your schedule, and in reality is not needed. Some software developers such as Spider Project developers in order to prevent users of using this aberration do not constraint the backward pass with the use of finish constraint but the constraint simply creates a finish flag and allows the software to continue with true CPM logic, the owner gets his flag, the schedule keep the true meaning of float.

Longest path is a concept all software should include, no doubt about it, finish constraints are a real life situation and you still have to deal with them by isolating their path from the rest of the network. An interesting situation might occur under limited resource scheduling where avilability of resources from activities not linked to the isolated path do enter temporarily into it. Don’t give up, keep asking for it.

Best regards,
Rafael

Carlos,

Beware that most definitions of Critical path are outdated, if not absolutely wrong. you cannot take out of your definition the true cause and effect.

Some definitions and many software avoid mentioning the fact that resource constraints can make your critical path to depend on resource availability and not merely on the traditional CPM relationships of the 1960’s.

You might have two activities not related by logic but by resource availability, many faulty software misses to the extreme they show available float on activities that if delayed will delay other activities not because of traditional CPM logic but because of resource availability. It is misleading, it is wrong and unfortunately some institutions I questions their credibility avoid the issue as if supporting some software vendors that cannot deliver a true answer.

Using soft relationships is a wrong dirty trick, that prevents you from showing true cause and might prevent you from unlevel the netework, add resources and find a better solution.

A few use the excuse that there is no absolute optimum solution. Can you imagine if courts start to require such level of proof, absolute perfection, no claim would pass the test.

Look for a definition that does not exclude real life constraints such as resource availability.

Suppose you are scheduling combing your hair and tying your shoes, none is dependent of the other any can happen first, but you cannot do both at the same time, your hands are a resource constraint you cannot overlook.

Best regards,
Rafael

Carlos

What’s your opinion on the definition in this recommended practice from AACE?

http://www.aacei.org/technical/rp.shtml -->
"COST ENGINEERING TERMINOLOGY"

Good luck!

Dieter