MS Project - Critical Path Dependant On Resources

[mtbtomo]

I have constructed a project plan (using MSP98) with no resources and formatted it for the critical path. A path appears that is from the start to the finish.

Now, if I allocate some resources then my critical path disappears appart from a few of the later tasks. A few summary tasks show when I filter for critical tasks but they are not shown in red like the critical tasks. If I double the amount of resources then my full critical path reappears despite not having too few resources in the first instance? Any ideas what is happening?

And should I link subsequent tasks to prior summary tasks? i.e. a whole group of tasks need to be complete before an individual action can be completed? I've heard its not good practice...

 

[Steve House]

"Critical Path" is one of those terms that has a technical definition that
can be far from what is meant in ordinary speech. The critical path is
simple that sequence of tasks that determines when the project will finish.
It may or may not run all the way from start to finish. The tasks on it may
or may not be really important to the success of the project. The sole
criterion of whether a task is critical is whether a delay in its completion
will delay the project finish. Period. It may be super important or it may
be totally trivial but if a delay in its performance delays the completion
of the very last work in the project, it is critical and on the critical
path.

--
Steve House [MVP]
MS Project Trainer/Consultant
Visit http://www.mvps.org/project/faqs.htm for the FAQs

I have constructed a project plan (using MSP98) with no resources and

formatted it for the critical path. A path appears that is from the start
to the finish.

Now, if I allocate some resources then my critical path disappears appart

from a few of the later tasks. A few summary tasks show when I filter for
critical tasks but they are not shown in red like the critical tasks. If I
double the amount of resources then my full critical path reappears despite
not having too few resources in the first instance? Any ideas what is
happening?

And should I link subsequent tasks to prior summary tasks? i.e. a whole

group of tasks need to be complete before an individual action can be
completed? I've heard its not good practice...

 

[mtbtomo]

Thanks Steve, but if there is a thread of linked tasks (irrespective of all the other unlinked ones) through my project, then wouldn't you expect the critical path to run from start to finish?

You may disagree on this but if I was to say that my first couple of tasks relate to the raising and distribution of an order - and nothing else can proceed until this has happened. So these tasks are under a summary task 'Stock Order' and all further tasks relating to the completion of the order are successors of this summary task. I would therefore expect, as it would delay the project, that at least the longest sub task of 'Stock Order' would be on the critical path.

Indeed, this is the case without any resources added. But when I add resources the CP disappears and starts mid-project. In the first instance, everything is still reliant on 'stock Order' happening.

If I then go into File -> Project Info and set 'schedule from project finish date then everything turns critical. Perversely, (if I reset the Project to schedule start to finish) if I double the available resources then my critical path reappears from project start to finish....

 

[Steve House]

Nope - consider this scenario. My project has a single thread of 4 tasks,
each lasting one week. They are linked FS in sequence. The project starts
1 Aug. Normally the project would end 4 weeks later, 28 Aug, BUT there is a
Start No Earlier Than constraint of 21 Aug on Task 3 because that's when the
parts needed to proceed with that task will arrive. Task 1 goes from 8/1 to
8/7, task 2 from 8/8-8/14, then there's a 1 week gap of inactivity, task 3
goes from 8/21 to 8/28 and task 4 from 8/29 to 9/5 (dates approx). You'll
find the critical path doesn't start until 21 Aug. with task 3, because
while 1 and 2 certainly must be done before 3 they *could* theoretically be
delayed up to the point that task 2 finishes after 21 Aug before their delay
affects the project's finish date. Thus they are not critical tasks even
though their work may be essential. OTOH, *any* delay to 3 or 4 will
immediately effect the project finish date and so they are critical from the
get-go.

In your case it sounds like a task in the middle of the project is being
delayed beyond it otherwise could start due to resource unavailability of
some sort. Perhaps he's on vacation or something. At any rate, that causes
the critical path to start with the first task he's to work on when he does
become available. The tasks before that date *could* be delayed to the
point that it eats up the time gap between when the task could begin if the
resource was available and when it is presently scheduled to begin before
they affect the project's finish date. If there's *any* delay possible,
even one minute, the task is non-critical until that delay is used up, then
it becomes critical.

Steve House [MVP]
Project Trainer & Consultant

Thanks Steve, but if there is a thread of linked tasks (irrespective of

all the other unlinked ones) through my project, then wouldn't you expect
the critical path to run from start to finish?

You may disagree on this but if I was to say that my first couple of tasks

relate to the raising and distribution of an order - and nothing else can
proceed until this has happened. So these tasks are under a summary task
'Stock Order' and all further tasks relating to the completion of the order
are successors of this summary task. I would therefore expect, as it would
delay the project, that at least the longest sub task of 'Stock Order' would
be on the critical path.

Indeed, this is the case without any resources added. But when I add

resources the CP disappears and starts mid-project. In the first instance,
everything is still reliant on 'stock Order' happening.

If I then go into File -> Project Info and set 'schedule from project

finish date then everything turns critical. Perversely, (if I reset the
Project to schedule start to finish) if I double the available resources
then my critical path reappears from project start to finish....

 

[davegb]

mtbomo,
Your thinking is correct. There have always been flaws in the critical
path analysis of MS Project, particularly after resource leveling.
However, there are requirements that must be met for critical path
analysis to occur correctly in any software. The most important is
Schedule Continuity. If your project is a series of groups of tasks,
some linked to others, but not all linked as a whole, you'll have
problems in getting or keeping a critical path throughout the project.
This is a little known but crucial requirement of CPM scheduling (it's
not even mentioned anywhere in the Project Management Institute Body
of Knowledge or in any MS manuals on Project that I'm aware of). \
As for everything going critical when you schedule from finish, that
is correct, because project automatically schedules every task to it's
Late Start date, which, by definition, schedules the task with no
slack, making it critical.

David G. Bellamy
Bellamy Consulting

Thanks Steve, but if there is a thread of linked tasks (irrespective of all the other unlinked ones) through my project, then wouldn't you expect the critical path to run from start to finish?

You may disagree on this but if I was to say that my first couple of

tasks relate to the raising and distribution of an order - and nothing
else can proceed until this has happened. So these tasks are under a
summary task 'Stock Order' and all further tasks relating to the
completion of the order are successors of this summary task. I would
therefore expect, as it would delay the project, that at least the
longest sub task of 'Stock Order' would be on the critical path.