Critical Path Breaks up!

Question

Hello MVPs,
A challenging problem, for me atleast.
To put it in simple terms:-
--- 10 tasks all linked to FS, 5 day duration, 1 resource, Standard calendar & Base calendar (THG_Cal-7 in file) of M-F 5d x 8hrs
--- All activities in Red critical path, so far so good, please see attached file.
--- Once I change the calendar of Act-5 in the middle to Cal-2 (20hr x 7d), the critical path breaks up!
My querries are:-
1. How does MSP calculates critical path, based on Project Calender, Elapsed time, Task calender?
2. Is it possible in MSp to allocate calenders to logical links like Primavera P6 or other scheduling software
3. Why MSP calculates Total slack as 6days for Act-5, which means based on Task calender not Project calender?
4. Lastly Whats the wayout to indicate correct critical path incase your programme has >1000 activities?
Your comments would be much appreciated.
Cheers!

Answer 1

Dominic Moss,
The idea that Critical Path iquals zero float is something that was created when there was found the first solutions to the problem: How long will my project take to be finished;
Later analysis discovered that are many situations not covered by the way CPM calculates FLOATS, based in the step-backwards from last date and only through a series of tasks connected between each other.
Therefore, in several situations - in special when there are resource leveling - MS-Project looses the ability to perform the Critical Path identification as it does exclusively with the Critical Path definition from the CPM.
I have been debating this topic for over 10 years! There is never a consensus.
However, there is a proof that the concept of Critical Path is broader than before when you see the late changes from PMBOK from 3rd edition to 5th edition.
Third Edition says:
“Critical Path. Generally, but not always, the sequence of schedule activities that determines the duration of the Project. Generally, it is the longest path through the project. However, a critical path can
end, as an example, on a schedule milestone that is in the middle of the project schedule and that has a finish-no-later-than imposed date schedule constraint.” (pag. 357)
So, it is what MS-Project performs and then in several situations you have a "broken" or "interrupted" Critical Path.
However, in Fifth Edition, a lot based in findings from the Practice Standard of Scheduling, we have:
“Critical Path. The sequence of activities that represent the longest path through a project, which determines the shortest possible duration.” (pag. 536)
The RULE that it must have zero float is taken away from this definition.
If you run MS-Project Add-ins that perform Resource Critical Path, you will find a Critical Path calculated with zero float BASED in resource restrictions, not task dependencies. This will solve 90% of the problem.
But you will also find external dependencies and some strange interactions of calendars and types of dependencies that will break both CPM and RCP calculations.
So, you will actually find certain situations in which the PROJECT Critical Path may have floats.
To fix these situations, there is an attempted rule from Primavera and there are other manual considerations that can be made, that are expressend in a document from AACE
AACE International Recommended Practice No. 49R-06
IDENTIFYING THE CRITICAL PATH
There, you will find FOUR different methods, in all we have commont the principle that the Critical Path is always the longest path, not necessarily the one defined by floats.
“There are several accepted methods for determining the critical path. The four most frequently used methods include the following:
Lowest Total Float – The activities with the lowest total float (sometimes abbreviated as “TF”) values may be considered critical path activities. This calculated value may be a positive number, zero, or even a negative number.  
Negative Total Float – Using this classification method, any activity that has negative float is considered critical, even if other activities have a lower negative float value. Under this classification, if an activity is contributing
to late project completion then it is worthy of inclusion on the critical path list regardless of its direct contribution to delayed project completion. Another way of framing this concept is to say that any work is understood to be critical if it scheduled
to be completed after the date required for a timely project completion.  
Longest Path – The concept and term, “longest path” has existed since the beginning of formal CPM theory. Commercial implementation of this process into a defined algorithm has only recently been made and is recommended for projects that
use multiple calendars and activity constraints.  The longest path calculation begins by identifying the activities that have an early finish equal to the latest calculated early finish for the project and then identifies all driving relationships for
these activities and traces them back to the project start date. This special calculation scheme is currently only instituted in Primavera scheduling software products. When discussing the theory of longest path, we will use the phrase using lower-case letters.
When describing the Primavera software  feature called, “Longest Path,” we will use upper-case to distinguish the implementation from the theory.
Longest Path Value Method - The principle of longest path value differs from the approach used by Primavera in that, instead of just noting activity inclusion or exclusion from the longest path, a numerical value is calculated for each
activity that describes the degree of longest path much like float describes the degree of criticality. Other principles are also enforced that will be described below. “
In other words:
- Use MS-Project with an add-in (Complete Critical Path can be found in Google)
- Use other tools (Spider Project, Primavera, and some others)
- Always double check the paths, and consider the activities with LESS float (Gregg´s response) in some parts of your schedule.
Regards,
Peter Mello, PMP, PMI-SP,SpS