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Question about P3’s CPM algorithm

5 replies [Last post]
Sat, 2008-05-17 07:45
Stephen Devaux
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I think I know the answer, but just to be sure, would someone be kind enough to tell me what P3 computes for the duration of the following six-activity project?

A 10d No preds
B 10d Pred: A/FS0
C 10d Pred: B/FS0
D 5d Preds: A/FS0, B/FF5
E 10d Pred: D/SS5
F 10d Preds: D/FS, E/FS

Also, what is the critical path, and what would the project duration be if Activity D’s duration was 9d? 3d?

Since this issue is not specific to P3, I am also starting a thread to discuss it in the Planning, Scheduling and Programming Discussion.

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Replies

Tue, 2008-05-20 07:10
#1
Sitaram Sakamuri
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Hai Stephen

I think the logic network is wrong. Every activity should govern by Start-to-Start relationship and Finish-to-Finish relationship i.e. it should start with ss relationship and finish with ff relationship or with fs relationship.

But in this the activity D is governed by ff relation ship and ss relationship i.e., just reverse.

So obviously you will land up in wrong algoritham.

Please clarify if I made any mistake

regards
sitaram
sitaram_75@yahoo.com
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Sun, 2008-05-18 11:03
#2
Stephen Devaux
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Whhops! Alex, I was about to point out that there were no "open end" activities in the network I posted. But first I decided to check one more time, and I discovered that I had made a typo!

Please forgive my error. The network should be:

A 10d No preds
B 10d Pred: A/FS0
C 10d Pred: B/FS0
D 5d Preds: A/FS0, B/FF5
E 10d Pred: D/SS5
F 10d Preds: C/FS, D/FS, E/FS

In the network I posted originally, I left out C/FS...

Sorry about that. But the problem remains the same. Now, with NO open end activites, the algorithms compute a project duration that is 5 days longer than necessary as a result of constraining the early start of D by its finish constraint.

At least, all the other algorithms I’ve seen do, and I’m asking if P# does the same.
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Sun, 2008-05-18 10:21
#3
Alex Wong
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Stephen

You seem to be everywhere ...

Well have to correct you again, software does not tell you have to built a correct CPM. It allows you to have open end doesn’t mean CPM can have open end activity. (A car is built to drive in any surfaces. However, if you drive in the walkway, its against the traffic law)

Good Luck

Alex
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Sat, 2008-05-17 15:09
#4
Stephen Devaux
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Ferdinand Uy wrote:

"first of all, lags should be less than half of its duration, you have too many open links using ss and ff links..the logic seems wrong"

I’m not sure what you mean by "the logic seems wrong". To my knowledge, there is nothing in any software limiting the number of SS and FF relationships, provided a logical loop is not created. There also are no activities in this project whose finish is unconstrained.

The fact that good practice would allow you to work around the algorithm’s problem is not the point -- the point is that the algorithm is unnecessarily adding time to the project duration through its failure to compute the ES of Activity D correctly. It’s pretty ridiculous that, in a simple network, the software is computing a schedule to be more than 10% longer than it needs to be. And it leads to the present case where:

1. The wrong activities are identified as critical.
2. Making a "CP" activity longer makes the project shorter, and vice versa.

Again, I’m simply asking in this subforum if this is also the case with Primavera, as it is with all the other software packages I’ve seen (and I’ve seen a lot!).

For simplicity’s sake, once it’s determined if Primavera does the same thing, I’d rather confine the discussion of it to the thread in the Programming and Scheduling subforum.


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Sat, 2008-05-17 10:50
#5
Ferdinand U.
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first of all, lags should be less than half of its duration, you have too many open links using ss and ff links..the logic seems wrong
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