Critical Activity and Longest Path

Dear Planners.

What is the diffeerence between defining critical activities as:

1- Total Float less than or equal to ....(e.g zero)

2- Longest Path.

Best,

Moutaz

Oracle Primavera - PM6
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Rafael Davila 👤 Member for 22 years 3 months
· Permalink

Under reswource constraints and resource leveling it is stll flawed, even if you could make P6 to disclose reliable resource leveled float values.

This is so because Longest Path Theory was devised before CPM of the 70's.  The workarounds cannot deal with more modern and advanced models (and a few old ones) that in addition to date constraints, multiple calendars and renewable resources also include leveling for consumable resources, spatial resources, financial resources and a long list of model enhancements not considered 50 years ago.

Have you ever wondered why the controversial AACE International Delay Analysis RP does barely mention resource constraining and gives no guidance at all with regard to this issue?  Maybe they are so much into Longest Path that they can't see the forest for the trees.

Resource allocation can affect the results of a delay analysis, performing a schedule analysis without considering resource allocations may increase the owner’s or contractor’s risk of assuming delay responsibility which is not his or her fault.

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Zoltan Palffy 👤 Member for 16 years 10 months
· Permalink

its the LOWEST total float value.

the 24 could mean hours which would be the same as looking at it as 1 day

H
Harish Anjannappa 👤 Member for 9 years
· Permalink

Hi,

In one of the programmes, contractor has made a P6 Setting for Define critical activities a longest path 24 hours. In the previous programmes he used different duration. what may be the reason for this. Is the programme acceptable. This is an NEC Contract.

I did awhat if analysis by defining the critical activity to 0 hour, the CP is shortened.

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Rafael Davila 👤 Member for 22 years 3 months
· Permalink

The Critical Chain is determined by Critical Activities, if the math is done correctly Critical Activities will have either a start total float or finish total float equal to zero. The manipulation that yields negative float distort the true float values, it is a mathematical algorithm that allows for the impossible, late dates earlier than early dates.

Critical Path calculations shall always consider resource constraints, date constraints and any other type of constraints, the issue is on using the right algorithms that yield correct early and late dates and not the impossible.

Calculation of Critical Chain shall always lead to longest path otherwise it is flawed. 

Usually the flaw is on how most software handle date constraints when unfeasible, they distort the backward pass and special software or calculations are needed to do it right.  It is fun to look at their selling of longest path as if Columbus just discovered America. 

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Rafael Davila 👤 Member for 22 years 3 months
· Permalink

http://dx.doi.org/10.4236/jbcpr.2013.13010

Understanding the Occurrence of Two Total Floats in One Activity and Schedule Crashing Approaches for That Situation

ABSTRACT
 
Critical Path Method (CPM) Scheduling has proven to be an effective project management tool. However, teaching the topic has proven it difficult to include all elements of CPM yet keep it simple enough for students to understand. In an effort to simplify the teaching of critical path method scheduling, the issue of two total floats in an activity does not get the attention necessary to address its occurrence. The objective of this paper is to present a mathematical method to show multiple total floats are possible for an activity. Also presented are suggestions for schedule crashing when multi-ple total floats are found. Two totals floats can be found if constraints (Lag or Lead) or non-Finish-to-Start (FS) rela-tionships, or both are used in a network diagram. Situations are possible where an activity may have a start total float (STF) of zero but have a finish total float (FTF) greater than zero, or vice versa. Because the critical path generally fol-lows the zero total float, these situations, where either the STF or the FTF is critical while the other is not, determine how the critical path activity must be controlled and crashed. This paper will present approaches of how to crash the schedule when a portion of the activity, either start or finish, is critical. Also presented will be methods to teach the subject matter with or without the use of scheduling software. Critical Path Method was revisited to see what the mini-mal conditions are needed to have activities with two total float. Generalized crashing methods were studied to see if the methods can be used when two total floats exist.
  • 1.2. Observation
    • When multiple relationships or a lag/lead time is used in a CPM diagram a situation can arise where there are two different total floats for an activity, start total float (STF) and finish total float (FTF). STF is the amount of delay at the beginning of the activity whereas FTF is the amount of delay at the end of the activity without delaying the whole project. When this happens, either the start or fin-ish dates of the activity, whichever is critical, must be followed because the critical path follows zero total floats. It affects how a project can be managed based on where the critical path follows.
  • 3.2. Comparison of Scheduling Software
    • However, because both P6 and MSP do not know what to do with the situation in Activity H they default to what they can do. 
  • If you do not consider start and finish dates the longest path calculation might be flawed.

From:  http://www.mosaicprojects.com.au/WhitePapers/WP1043_Critical_Path.pdf

  • Critical Path: The sequence of activities that determine the earliest possible completion of the project, or a phase of the project. This new definition12 does not preclude the possibility of several ‘completions’ within the one project to account for interim handovers required under a contract. It allows for the possibility of the critical path starting at the beginning of the schedule or at some interim point where an external dependency allows the ‘critical’ work to start. Additionally, the sequence of activities may be determined logically (through links or dependencies) or through the sequential movement of resources. The definition is both concise and unambiguous. This definition has been included in ISO 21500 Guide to Project Management, as: Critical Path: sequence of activities that determine the earliest possible completion date for the project or phase. 
  • This definition allows for the possibility of activity start/finish be on the path, allows for the possibility of Occurrence of Two Total Floats in One Activity.

From:  http://www.ronwinterconsulting.com/Longest_Path_Value.pdf

  • This reference can give you an idea on how longest path is calculated by some software.
  • I have my doubts that under complex resource assignments this procedure has been correctly implemented. Well it looks that even for simple schedules neither MSP and P6 got it right. 
  • Longest Path equal path with 0 float when using software that can calculate negative float separate from total float simply by making last activity late dates equal to calculated early dates when the constraint that causes the negative float is unfeasible. 
  • Under resource leveling it is known as Critical Chain.
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Zoltan Palffy 👤 Member for 16 years 10 months
· Permalink

remember the critical path DOES not have to be 0 it can also be a positiive or negative number.

so I can have the longest path thru the project with 10 days of foalt on it 

or I can have a critical path wih 10 days of float on it also 

the critical path has the least amount of float it can be positive, zero or negative.

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Gary Whitehead 👤 Member for 17 years 2 months
· Permalink

total float is exactly what you would expect -looks at the total float field

 

The longest path is calculated by identifying the activities that have an early finish equal to the latest calculated early finish for the project and tracing all driving relationships for those activities back to the project start date.

 

Longest Path can differ from Total Float = 0 if you have multiple calendars, or hard constraints which artificially affect an activity's total float.

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