Total Slack Value + Elapsed Days

[Tito]

I'm having a problem with elapsed days and negative slack values. The total
slack values calculate correctly for Edays whenever you have positive slack.
Once I change the constraint on the completion date to create a negative
total slack value, the Eday calculation is wrong.

Example -
1. create a simple string of tasks with each task 5 days in duration.
2. Add a constraint to the final task in this string which causes your total
slack value to be negative.
3. Change any one of the durations from 5d to 5ed and you'll see the
negative slack value get larger instead of smaller i.e., -25.38d instead of
-20d like all of the other tasks in that chain.

I actually expected the total slack to decrease below -20d....

 

[Jack Dahlgren]

Some of the calculations with edays are a bit weird. I think this is for two
reasons:
First, using edays puts the task finish and start at the very beginning and
finish of the day (ie: midnight instead of 5:00PM) The second thing is that
the calculation for what is actually a "day" within project assumes 8 hours.
When you use an eday, a day is really 24 hours.

There is a third reason which I suspect but have not fully investigated and
that is that there are some bugs in the way edays are interpreted. By bugs I
mean that the behavior is not consistant in all calculations. This is just
suspicion on my part and I don't really care that much about it to bother to
spend time investigating and reproducing.

I think that either of these may explain the situation, but they don't
really help. You may get better results setting a 24 hour task calendar for
the tasks you want to use edays for.

-Jack Dahlgren

 

[Tito]

Thanks for the quick reply, I appreciate it. I tried using the 24 Hour
calendar but things get hosed when you're dealing with negative slack
values....the obvious question is "why do you have a negative slack value in
your schedule?"...we're working on it. Just thought the eday calculation was
strange and wanted to see if anybody else saw it too

 

[Steve House]

Negative slack occurs whenever a task is either driven to finish after a
date that has been entered into its Deadline field or when a "brick wall"
constraint (SNLT, FNLT, MSO, MFO) prevents a task from being scheduled where
its predecessor links say that it must be scheduled. IMHO, negative slack
always indicates either a project doomed to failure to meet its required
completion and/or a schedule that is unworkable due there being a physical
impossibilty in the task seqencing (ie, a MFNLT constraint that forces a
milestone to be displayed as occuring on a date that is earlier than the
date on which its predecessor finishes). I believe deadlines should be used
to express objectives such as required completion dates while constraints
are best used to model physical realities affecting the schedule coming from
factors other predecessor relationships and resource availability.
Deadlines reflect what we ought to do, or must do for a project to be
successful. Constraints reflect what WILL happen in the schedule no matter
what we do or even if we do nothing at all.