Tips on using this forum..

(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?

(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.

Non-critical activities showing up as critical path

9 replies [Last post]
Tammy Pitcher
User offline. Last seen 7 years 48 weeks ago. Offline
Joined: 11 Jan 2011
Posts: 9
Groups: None

Good day,

I built this little schedule, there are only 6 activities, no constraints, everything is pretty much FS, but for some reason the activities are showing up as critical path! When I click on 'schedule' there is an option tab, I have gone in there and changed the Define critical activities and everything is still coming up as critical path!!??!!??!!

Any in-sight would be great!!! Plz & thx :)

Cheers,

Tammy 

Replies

Rafael Davila
User offline. Last seen 5 hours 32 min ago. Offline
Joined: 1 Mar 2004
Posts: 4780

Miklos,

"So I think these guys a kind of mathematician or industrial engineers and never have encountered with applications that can handle calendars."

It is no surprise to me at all.

I did not paid attention to the solution on the article but to the existence of the problem and the possibility this and many similar problems cannot be modeled with the limited functionality of traditional CPM models. Because I am an end user and not a researcher I rarely pay attention to theoretical solutions that are not available on commercial software.

About MSP I have not found the answer regarding if out-of-the-shelf MSP provides the computation of Longest Path as well as for any other except Primavera and that not all Primavera implementations are the same.

http://scheduleanalyzer.com/sa_long_theory.htm

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

I believe under resource constraining the computations by P6 might miss the many hundreds or thousands of possible parallel longest paths. While multiple parallel longest paths are rare under logic only, on networks driven by resource constraining it is just the opposite. If you have at a given moment 40 activities demanding one unit of a resource for which 10 units are available the 40 are equally critical although only 10 can be performed simultaneously,  if these 40 activities are not linked by logic you will get a huge amount of possible permutations (order is relevant) that can result in a huge amount of parallel longest paths and I wonder if traditional longest path computations might be able to handle this. The only way to figure out if these software are flawed is by testing them.

I do not use Primavera products but have a license of the most sold CPM software in the world, MSP, therefore I want to find out how good it is at performing the longest path calculations under resource leveling, if it can. I rarely use MSP except for limited uses therefore cannot say 100% it does not have Longest Path functionality that works under resource leveling or even without. I can tell it misses resource critical float and seems like do not even try to figure it out.

This is also related to my argument that the AACE International 29R-03 is biased in favor of Primavera products if the availability of commercial software that implements longest path calculations is limited to Primavera or to a very small set of the hundreds of scheduling software, leaving the majority or a significant minority out. Here it is not like in the exampe I provided, it is expected that the experts did it right and considered the availability of the Longest Path calculation in commercial software, an obvious concern you not need to be an expert to realize.

Best regards,

Rafael

Miklos Hajdu
User offline. Last seen 6 years 40 weeks ago. Offline
Joined: 13 May 2011
Posts: 97
Groups: GPC Malaysia

Rafael,

I went through the article you offered me this morning. These guys 'invented' the so called

  • time-window constraint
  • time-schedule constraint
  • and the time switch constraint

Maybe that I was too fast, therefore I did not understand their invention well but I have the feeling

that

Time window constraint is nothing else than applying a start later then and finish earlier then constraint at the same time to the activity

Time schedule constraint is nothing more than a calendar

and time switch contraint is also nothing more thn a calendar e.g. weekdays are the work windows wekends are the rest windows. Just let me quote from their paper:

"Based on the analysis and achievement of [15], in this paper we assume that time can be treated as repeating cycles where each cycle consists of two categories: (1) some pairs of rest and work windows; and (2) a leading number specifying maximal number of times each pair should iterate. In this context, activities can be executed in a work
window, while activities can not be executed in a rest window. We name this kind of time constraint
in an activity network as time-switch constraint. "

Anyway I've found another very basic mistakes as well. They are talking abvout activity on arrow graphs. Well, precedence diagramming can be drawn as an activity on arrow graph a node represent the sart, an another the finish of the activity, but as ES+t=EF than this should be modelled by an arc with t length from the start to the finish, and by an another from the finish to the start with -t length.  

So I think these guys a kind of mathematician or industrial engineers and never have encountered with applications that can handle calendars.

Something else.... Did you find the solution offered by MSP for your problem?

Miklos

Rafael Davila
User offline. Last seen 5 hours 32 min ago. Offline
Joined: 1 Mar 2004
Posts: 4780

Miklos,

The following article presents a real and practical situation that happens in many real life jobs, a time constraint called time-switch constraint which assumes that an activity begins only in a specied time interval of a cycle with some pairs of exclusive components.

http://140.128.95.1/bitstream/987654321/1635/1/%E6%A5%8A%E6%97%AD%E8%B1%AA%20J17.pdf

This time-swich constraint I believe can be modeled in occasions with the application of true shift modeling available in Spider Project using shifts to represent full season cycles instead of weekly shifts in combination to the ability for activities to create resources as an activity moves and not fixed with time, another more direct approach might be with conditional scheduling functionality using switch activities or switch available at the lindividual link level. In other occasions it might be the use of financial constraints.

I doubt traditional longest path calculations can correctly determine longest path under the presence of time-switch constraints, a real model necessity in many practical applications, same as minimal-maximal relationships.

Use of constraints are a real necessity, the most notable is resource constrained schedules, it would be wrong to rule out the use of constraints just because it makes it easier to see and justify the logic. My hands down approach is to run the model and observe.

Best regards,

Rafael

Miklos Hajdu
User offline. Last seen 6 years 40 weeks ago. Offline
Joined: 13 May 2011
Posts: 97
Groups: GPC Malaysia

Tammy,

I agree with the others.

if you have only one path (from start to finish) then this will be the critical one. Of course there are different ways to define critical path, but the most common is, that criticasl path is the longest from the start to the finish.

As bacward calculation usually starts from the project duration (this is the length of the longest path) therefore these activities has  zero float. This is the basic in every software I know. There are some intereting scheduling options that can modify this, but this is an another issue.

One last comment: I agree with Mike, avoid the use of constraints. A network should based on the logic, and not some user defined constraints. Of course in some cases you can not avoid them for example when you have contractual milestones, and you have to arrange your schedule according to these.

Miklós

Mike Testro
User offline. Last seen 1 week 8 hours ago. Offline
Joined: 14 Dec 2005
Posts: 4393

Hi Tammy

You do not need constraints to create a critical path - leave the constraints off and the software will show the unrestricted critical path.

This is why you should NEVER use constraints in any programme.

Best regards

Mike Testro

Muthukumaru Senth...
User offline. Last seen 7 years 3 weeks ago. Offline
Joined: 9 May 2011
Posts: 25
Groups: None

Hi ;

 

As above mentioned , critical path is;

1: Longest path

2: Zero float

3: User defined critical path depend on the project( 0 days, 10 hrs , 10 days, -20 days, -50days etc)

Here if you do not want to see the critical path in your programme , as you may have some float to finish the project, set the programme critical path under the category 3 , it means define your float to calculate the critical path.

 

Senthuran

Gary Whitehead
User offline. Last seen 15 weeks 2 days ago. Offline

Tammy, I'm not sure what software or strange settings you have been using for the past five years, but this is basic critical path stuff.

 

For every programme I've ever seen on every software package I've ever used:

 

red means critical means toital float = zero.

 

total float = zero means if the activity is delayed by even a day, it will delay the entire project.

 

If you have no constraints, the longest path will be the critical path.

Tammy Pitcher
User offline. Last seen 7 years 48 weeks ago. Offline
Joined: 11 Jan 2011
Posts: 9
Groups: None

Hi Steve,

I understand what you mean, but generallly if I do not give the project a 'finish by' date everything is green. I changed the setting for defining critical path and that worked.

Just now I was updating a template, adding milestones but not changing any dates and after I rescheduled, some activities showed up red!!!! If there is no finish constraint it should be green! I have been scheduling for just over 5 yrs and this is the 1st time I have come across this issue.

Cheers,

Tammy

Steve Campbell
User offline. Last seen 5 years 30 weeks ago. Offline
Joined: 6 May 2011
Posts: 6
Groups: None

Tammy,

If you have only six activities they are likely just on one path so there is no non-critical path.  All you have there is critical.

 

Regards,

Steve