Adjusting duration based on constraints that are variable..

[Homer]

My problem is a very basic one but, I don't know if there is a way to handle
it with MS Project?

Let's think that there is an activity which will start with the start of
another activity. So they should be related SS.
This activity should also only be finished when another activity is
finished. So they must be related FF.
However, if these are not only relations but also constraints for the
duration of our activity then there must be a way to assign these relations
as constraints. It is different than other constraints in MS Project which
can only be set up by date.

The duration in between should be calculated by MS Project automatically in
the backward pass. But, the duration field always waits something from me..

 

[JulieS]

Hi Homer,

I believe you are looking for a hammock task. See FAQ #19, "Hammock
Tasks" at:

http://project.mvps.org/faqs.htm

I hope this helps. Let us know how you get along.

Julie

 

[Trevor Rabey]

I think the interpretation of what the links mean is the start of the
difficulty.
Try to be very clear about the "condition sentence" that you apply to each
type of link.
So "will start" and "should also be finished" are a bit ambiguous.
If you are scheduling forwards from a start date and all tasks start asap
(Project, Project Information) (good CPM default, no need to change),
then your sentence is "Task B cannot start until Task A starts" for the
first predecessor link.
The sentence for the next one is "Task B cannot finish until Task C
finishes", and Task B is the same in both cases.
I need to explain the next part by example.
Suppose there is 10 days between the start of A and the finish of C. B can
go anywhere in that gap provided it has duration less than or equal to the
10 day gap, and the FF link will drag the F of B to the F of C, but the
duration is indeterminate, within 0 -10 days, to MSP.
If the duration is more than 10 days, well that's just a conflict, both
conditions can't be met.
You can't use these to drive the duration of B, which is indeterminate, but
anyway predecessor links won't drive durations in any case.
To make B's duration expand and contract between the limits, you need to use
copy/paste special/links, with the start of A copy/paste specialed to the
start of B, and the finish of C copy/paste specialed to the finish of B.

 

[Homer]

Yea, w/ hammock something can be done but, it is not what I want.

I have written SS and FF to simplify the situation. But, if the activity is
should start after 2 days, namely SS+2, and should finish after 10 days of
the other activity, namely FF+10, then we cannot do this operation by paste
special.
Or can we?

 

[Homer]

To overcome these difficulties what I do is actually to create two dummy tasks:
One for the start of the activity and the other for the finish of the
activity.

I link the start dates of these dummy activities to the related activities
and give durattions to assign the lag.

Finally I relate the finish dates of the dummy activities to the activity I
want.

But, of course, it could be much easier if we could assign a task directly
as a hammock, milestone etc. task by use of a drop-down menu and then assign
as many relations as we want.

For example: An activity of this kind might start conditionally after two
different activities with two different time intervals (e.g. one 18SS+3d the
other 91SS+5d) whichever is the latest.
Also for finish times similar conditions might exist.. We should be able to
tell these conditions to MS Project easily)

 

[Steve House [MVP]]

Remember that predecessor links describe permissive relationships rather
than obligate relationships. If tasks A & B are linked SS, the "SS" does
not mean the the start of task A *causes* task B to start, but rather it
means that the start of task A is what *allows* task B to start. It might
not start then but it cannot start any earlier, in other words. Same with
FF - the finish of A defines the earliest date that we could possibly
consider B to be done - B may take longer than that but it will never finish
sooner. It sometimes helps to remember too that in a "best practices" plan,
each task ALWAYS describes a physical activity that results in the creation
of an observable and measurable deliverable. As such, the task is only done
when that deliverable has been created and its duration is the precise time
that it takes to create it, no more and no less. Some administative types
of activity (such as project management itself) might be an exception but
they're relatively rare. To say that the task in question starts when
another task starts and ends when a third task ends implys that the exact
quantity of the deliverable the task must produce is somehow dependent on
the timing of the other two driving tasks. I'm not saying something like
that is impossible but it really is a relatively rare event.