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Creation of negative float, when end date’s is zero

2 replies [Last post]
Wed, 2006-02-15 06:09
Robert Burns
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Hi, I am experiencing problems with Version 5. Primavera are assiting, but the problem is rather convoluted, but to place it in simple terms - The programme is displaying a negative float path AFTER, the zero project critical path, such that the project end date displays a zero total float, whereas within 8 activities further up the backward pass , it is superceded by a -4 total float path. The client intitially created this with ’ladder’ logic and as the updates were non-sequential, I am scheduling to ’progress over-ride’. Is the fact that a) - the ladder logic is constraining the activties (SS +15, FF +15) such that the already critical activity is now beyond a sensible arithmetical calculation, seeing as it began 4 months ahead of sequence (note the SS & FF on said activity)? b) - there is an error with the software ( i doubt this as I have ran a diagnostic)? c) - could the ’Purge Deletes’ function have an impact, as when I re-schedule the -4 becomes - 9 weeks? ..............simply put, it doesn’t make arithmetical sense for this calculation to exist, and the software is having a nightmare of a time trying to cope with it all. I did take out all ladder activities, which resolved the total float path, BUT it still irks me that the problem occured and it also irks that we have still to resolve why this occured. All advice and thoughts will be met with joy on my part. Thanks, Robert
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Replies

Tue, 2006-02-21 10:12
#1
Robert Burns
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Many thanks for the input Ronald. I did go back and chase up the ’dominant’ logic which was placing the constraints, giving the negative float. However, i am still bewildered as to why the negative float path occured, when I have been informed by primavera that the ’progress over-ride’ should not produce such calculations.

I can only, for the moment, conclude that the out of sequence activity, which had a SS +15 & FF +15 predecessor , was so tightly constrained but had an actual placed at such an out of sequence date, that the software could not appropriate a sensible calculation (after all in the real world, the change would have been noted, a new logic placed and then re-scheduled).

Many thanks for your thoughts once again.

Robert
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Wed, 2006-02-15 09:35
#2
Ronald Winter
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Off the top of my head, I would say that what you saw was the natural consequence of using the Continuous Activities CPM option and a mis-matched doubly-constrained activity. The logic constraining the start of the activity (perhaps Start-to-Start?) plus the activity’s duration did not match the logic constraining the finish of the activity (perhaps Finish-to-Finish?). One of the two constraints needed to be overridden and Primavera always chooses the finish constraint as the dominant one. This caused the activity to start 4 days later than logic says that it should (and hence, the -4 float value. This also assumes that you are using the Start Dates to Compute Float option.)

I forgot – P3e/c does not have an option to choose Continuous versus Interruptible activities for the CPM calculation. You always get Continuous. P3 has this option which will automatically stretch the activity duration to prevent this sort of thing.

Congratulations on your curiosity and diligence at actually reading the schedule and using your head. That is what makes for great Schedulers. Good luck!
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