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Start slack, finish sack and critical path

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Saurabh Lele
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Hi everyone, I'm attaching a snapshot of a part of my schedule.I've highlighted few summary tasks which are showing "critical" in MSP, but none of its child tasks are critical. When I further checked, I found out that the "start slack" for those tasks is "0" whereas "finish slack" is having value. Usually for all the "non-started" tasks the start slack and finish slack are the same. Because the "start slack" is "0" for those tasks, maybe thats why they are being highlighted as "critical". I still don't know why it should happen like that.  image.png

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Saurabh Lele
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Thanks for the detailed explanation Tom.


Appreciate it.

 

Saurabh Lele
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Haha.

 

Could be.

 

Rafael Davila
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Saurabh Lele,

I agree with you, there is an error, perhaps By Design making it a Monumental Error.  There is no excuse for not to roll up corectly the dates no matter what logic is used, can be if using links or if using date constraints.  A 1950's scheduler would roll up the dates by hand and would get the correct results!

Dates_on_Summary_Bars

Some call it a Bug,  ... quite a BIG BUG.

Rafael Davila
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For your convenience here is query_2 image I saved before disappearing as I had the suspicion it will soon disappear.  I posted the image using external image hosting service. 

query_2

Tom Boyle
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Saurabh Lele, thanks for the graphics.

"I still dont see a reason of the summary task being shown as a critical sumary task."

Repeating myself:  The summary task is shown as critical because its TS=0 (and your schedule options say "Critical" ~ TS<=0).  Because the underlying calculations are not based on valid network logic, Total Slack and the critical flag of summary tasks in MSP are meaningless and should be ignored. 

 

"There is some error in the calculation of Late start of the summary task."

As noted in the article I linked earlier, if the early start of a summary task is controlled by a subtask that is manually scheduled, then MSP over-writes the late start of the summary with the early start of the manually-scheduled subtask.  I don't know why - you can call it an error if you like, though "correcting" the error would still not make the summary values meaningful.  Regardless, manually-scheduled tasks are 100% incompatible with logic-driven schedules.  They corrupt the float/slack calculations of all related activities, and their mere existence in your schedule is an error.  

Saurabh Lele
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Thanks for the article and your input Tom.

Yes, as you predicted the first sub task of "Preparation of PR" is manually scheduled.

But, unlike the summary task, both the sub-tasks have non-zero start and finish slacks.

I still dont see a reason of the summary task being shown as a critical sumary task.

 

Al

6097
query_2.jpg

so attaching the snap with Early and late Starts and finishes based on the calculations shown in your article.

There is some error in the calculation of Late start of the summary task. (Check attached image)

Saurabh Lele
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Attaching the jpeg image.

6096
query.jpg

Tom Boyle
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Saurabh Lele,

For Summary tasks in MSP - Start Slack, Finish Slack, Total Slack, and the Critical flag are meaningless and should be ignored.  They are computed from the Summary's early and late dates, which typically role up from different sub-tasks without regard to logic.  The particular example you've described (without reference to the invisible image) - with zero start slack and positive finish slack on an unstarted summary task - might be explained by a manually-scheduled subtask.  I wrote an article on the subject a few years ago: https://wp.me/p6CCB4-4A

Rafael Davila
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The image is not visible.