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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.

Unexplainable Total Slack

17 replies [Last post]
Wed, 2010-03-24 06:46
Stuart Atkinson
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Groups: Civil Engineering
For example, Why does a task with total slack of 3 days not go critical when I delay the finish or increase the duration by 3 days? In some instances it actually increases the ’total slack’.
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Replies

Mon, 2010-03-29 11:30
#1
Stuart Atkinson
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Groups: Civil Engineering
Sorry about the late reply. But thank you all for the effort you have put in replying to my question.
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Wed, 2010-03-24 16:57
#2
Mike Testro
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Groups: Building Information Modelling, Content Moderators, GPCCAR Editors
Hi Rafael

Last time I saw a beaver like that it cost me 250 bhat and I was robbed.

Best regards

Mike Testro
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Wed, 2010-03-24 16:51
#3
Rafael Davila
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Groups: Spider Project Team
Mike,

Thanks. Hope you won’t mind if a beaver.

beaver
A proud rat,
Rafael
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Wed, 2010-03-24 13:40
#4
Rafael Davila
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Well this is how HTML looks.

HTML Code
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Wed, 2010-03-24 13:30
#5
Rafael Davila
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Got him - he has croaked.

See Thread Nr 9 & 14

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Wed, 2010-03-24 13:23
#6
Rafael Davila
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Mike or PP Admin,

Please after fixing my post no 8 delete all my subsequent posts as to give back this page to Stuart Atkinson.

No Chance sunshine - we are revelling in your embarasssment.

Best regards,
Rafael
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Wed, 2010-03-24 13:13
#7
Rafael Davila
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Groups: Spider Project Team
May a new page be created!!!











.....
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Wed, 2010-03-24 13:10
#8
Rafael Davila
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I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.





I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.




I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.





I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.
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Wed, 2010-03-24 13:09
#9
Rafael Davila
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I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.




I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.



I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.
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Wed, 2010-03-24 13:09
#10
Rafael Davila
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I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.



I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.




I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.
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Wed, 2010-03-24 12:58
#11
Rafael Davila
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Well seems like an incomplete posting of html code resuls in an error on the hole page instead of just in my posting. Probably this is an issue PP Web developers can deal with as to limit the display to within the posting frame or even better to detect the error and just post the incomplete code so the person doing the post can access it and fix it.

I believe from prior observations that the error stop at the page andif you continue with nwe postings that create a new page it will stop.


Mike, can you delete the incomplete HTML code from my prior posting?

OK This once only - But your dancing frog gets it.
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Wed, 2010-03-24 12:21
#12
Bo Johnsen
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Hi Rafael,

One more time, could you please explain how you "attach" these screenshots? I know it doesn’t waste any space on PP-servers as it is basically only a link to some other remote server, but...how?


Regards,

Bo
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Wed, 2010-03-24 12:01
#13
Rafael Davila
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PDM Logic

This kind of logic I have seen to be very common on MS Project users, I call it "M.T. Logic", any similarity to a PP member name is pure coincidence.

The problem starts with the PDM computation on Early Start of activity B, P3 solve the issue by allowing splitting. Then after the activity consumes what I call "intermittent float" (don’t know if the term already exists or if other name is used) any further increase in activity duration start consuming Total Float.

A few days ago I found this article related to this issue.

http://www.pmicos.org/topics/aug2006tom.pdf
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Wed, 2010-03-24 12:00
#14
Bo Johnsen
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Hi Stuart,

The problem is most probably related to a FF-link somewhere in your schedule as Mike also indicates. MSP operates with start slack and finish slack and the smallest of those 2 equals the total slack.

That means that an activity can have start slack of 0, thereby being critical, but maybe a finish slack of 5 days and the total slack will be shown as 0. If you increase the duration with 3 days, I would as you expect a delay of 3 days as the total slack = 0, however, no delays happens, just a change in finsih slack from 5 days to 2 days.

Unfortunately, MSP doesn’t (at least my version) show the correct values of either start slack or finish slack, it always makes one equal the other and thereby equal the total slack, i.e. all 3 have the same value. Maybe a have a wrong setting somewhere.

Anyway, the increase in total slack when increasing a duration (sounds contradicting) can happen when you have a FF-link somewhere in the middle. A small test in MSP: Say you have 4 activities, a(8days), b(8days), c(8days) and d(20days). b is linked from a with a FF+6days link, c to b with SS+3days link. d not linked to anything, but having same start date as a. Both a, b and c will have total slack of 3 days.

Now increase the duration of b from 8 days to 10 days. You would expect the total slack to go from 3 to 1 day for some of a, b or c. Unfortunately, it will increase from 3 to 5 days for a, b and c, even though you have increased a duration. The overall finish date is still the same, determined by d’s duration.

Therefore, don’t always trust the total slack value in MSP. Use e.g. Pertmaster or P6 instead as they clearly distinguish between Total Start Float and Total Finish Float. Powerproject only has Total Float (to my knowledge), based on the Finish Float (and that is all one need if only setting up programmes with FS-links).


Regards,

Bo
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Wed, 2010-03-24 10:08
#15
Mike Testro
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Hi Stuart

Are you using multiple calendars - for instance a task set at 24/7 could end within a non-working period.

Are you using SS FF links with lead/lags?

When you move/increase the non-critical tasks what happens to the successors when you re-schedule? Do they move by the same degree?

Best regards

Mike Testro
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Wed, 2010-03-24 07:45
#16
Stuart Atkinson
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No Gary,

incidentally, this occurs on numerous tasks.

i do have ’calculate multiple critical paths’ option on.
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Wed, 2010-03-24 07:08
#17
Gary Whitehead
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Does the activity have any sucessors, constraints or deadlines linked to it’s start rather than finish?

That’s the only reason I can think of.
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