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Critical Path - Longest Path won't show as critical

7 replies [Last post]
Kyle S
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Hi All,

I'm working on a schedule that has a very generous duration and anticipate finishing ahead of schedule.  Therefore my schedule has a lot of float and no activities show as critical.  I'm attemting to use the longest path. However when I select longest path and schedule no activities show as critical. 

After scheduling with the longest path, I filtered by longest path and the only activity that shows up is my substantial completion finish milestone.

Most activities are task dependent with start and finish milestones. 

Any ideas why it won't the longest path as critical?

Thanks.

Replies

Patrick Weaver
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Float does not define the critical path (see: http://www.mosaicprojects.com.au/WhitePapers/WP1043_Critical_Path.pdf), but most software ignores this.

Solving your problem depends on the tool, some systems allow you to define the float value that shows as critical, others allow constraints that set the float on activities prior to the designated event to Zero.

From a communications perspective you do need to be ale to focus people on achieving the quickest reasonable completion time. 

Tom Boyle
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Kyle,

Re. your last question...

Early constraints (start/finish on or after) are legitimately used in lieu of logic only when they represent the product of work that is truly external to the current project.  For example, if a construction contract includes installation of Owner-furnished material that the Owner promises (in the contract) to supply by the 1st of July, then an "Owner Material Ready" finish milestone with a FOA constraint of 01Jul'16 17:00 might be appropriate.  As a consequence, if the contractor has scheduled all his preparatory work to finish a week before arrival of the owner-furnished material, then those works have a week of float, and there will be no driving relationships between the contractor's preparatory works and the installation.  The "Critical Path" (or Longest Path) is truly and correctly truncated at the constrained milestone; it is what it is.  

I would suspect that many or most of your early constraints are used in place of logic (that is activities, durations and relationships, all internal to your project) that has simply not been defined.  You need to communicate with the stakeholders and determine what the missing logic is - i.e. why are these activities not starting at the beginning of the project?  Is the detail design and submittal process scheduled?  Is the material procurement and delivery process scheduled?  Is the mobilization and deployment of resources (manpower and equipment) scheduled?  I would not suggest using Start to Finish relationships (with or without lag) in any case. 

At the end of the day, your project might indeed only have one short "critical path" that is truncated by a legitimate early constraint, accompanied by a number of parallel near-critical paths.  You can use multiple float path analysis to define and display these.  (Here's something I wrote about MFP few months back: http://wp.me/p6CCB4-L).  The method Zoltan describes in his recent post is essentially a manual version of what P6 does automatically with MFP, though his description only exposes the first float path.

Good luck, tom

Zoltan Palffy
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Kyle

Another work around is to leave what you have and create a new code call it CP for critical path

Now make the new code visible as a column

filter for your last activity now look at the predecessors that are driving and critical and click on goto whne you get back ot the data date these activites will be your critical path.

Now in the criticla path code column assign it the cp code

now do a fill down in the cp code column

now you can filter for cp 

 

Kyle S
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Thanks for the responses.  

Tom, you were correct I had restained the substanital completion milestone to the contractual date.  I removed this constraint and now have a handful activities that show as critical.

I believe the issue is that I have used a fair amount of "start on or after constraints".  The project consistis of work is many different areas of a large site and much of the work will be able to be done concurrently and is not driven by the completion of another tasks.  

In this situation how would recomend revising the logic to remove the start on or after constraints yet still control the approximate start date of the activities? 

Start to finish w/ Lag? 

Thanks again for the input.

Tom Boyle
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Hi Kyle,

Longest Path identifies the logical driving path to the last activity in the schedule, i.e. your completion finish milestone.  I would guess that you have constrained this milestone to keep it on the contract date, so none of its predecessors are driving it - thus no Longest Path.  Remove the constraint, and the Longest Path will appear.

 

Zoltan Palffy
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if this is the case you can change the value or the threshold to increase the definition of critical. Lets say that the least amount of float or longest path is 10 days. The default in the program for defining the critical path is set to 0 days.

To change the definition of critical based on the number of days of total float there are 2 places where you can do that one is at the project level and one is when scheduling options (F9).

 To change it at the project level click on the Projects directory on the left

then click on the setting tab 

then at the bottom under Define Critical Activities 

select total float less than or equal to 

and then on the right change the 0 to 10 or what ever your lowest float value is.

To change it in the scheduling option

select tools schedule

options (lower right)

then in the middle of the next page under where it says Define Critical Activities 

select Total Float less than or equal to 

and then under that change the 0 to 10 or what ever your lowest float value is.

Rafael Davila
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http://docs.oracle.com/cd/E17462_01/Web_Access/Help/en/Projects/Tasks/pm...

Longest path defines the sequence of driving activities that determine the project end date.

The longest path is broken when activities are no longer driven by relationships; that is, when activity dates are driven by constraints or resource leveling.