J
Jamie Hart
How does MSP determine the critical path? What is the criteria?
S
Steve House
The critical path is that sequence of tasks that determines to overall
project duration. First of all, all tasks in the project except for the
start and the finish milestone MUST have both a predecessor and a successor.
If we have parallel chains of tasks through the project, all paths must
converge on the finish milestone if they haven't joined up before. One of
those chains will be the longest and that is the critical path. Technically
each task has a property known as slack time or float. The slack time is
the amount of time a task could be delayed before it delays a: a successor
task, or b: pushes a task past a stated deadline, or c: the project finish.
If the slack time is zero the task is critical - any delay to it will also
delay something later in the project. The critical path is the chain of
critical tasks from project start to project finish.
project duration. First of all, all tasks in the project except for the
start and the finish milestone MUST have both a predecessor and a successor.
If we have parallel chains of tasks through the project, all paths must
converge on the finish milestone if they haven't joined up before. One of
those chains will be the longest and that is the critical path. Technically
each task has a property known as slack time or float. The slack time is
the amount of time a task could be delayed before it delays a: a successor
task, or b: pushes a task past a stated deadline, or c: the project finish.
If the slack time is zero the task is critical - any delay to it will also
delay something later in the project. The critical path is the chain of
critical tasks from project start to project finish.
J
JackD
Jamie Hart said:
The basic critical path calculation is simple. Start at the first task, find
the finish date, go to the successors of that task, add their duration to
the finish date of the predecessor - this is their finish date. Do this for
every task. You will get the "early start" and "early finish" for each task.
Step 2 is to start at the end. Take the finish of the latest task, subtract
the duration of that task (getting the late start) and then the same for all
of the predecessors etc. This gives you the late start and late finish dates
for each.
Finally, compare late finish to early finish. The difference between the two
is called Float or Slack. If slack is 0 (that is that the late finish and
early finish are the same, the task is on the critical path. You can not
change the duration of that task without affecting the end point of the
project.
D
davegb
Just to further clarify the above descriptions:
Free Slack is the amount of time a task can slip and not effect any
other task.
Total Slack is the amount of time a task can slip and not effect the
end date.
Total Slack is calculated as described above. Free slack is a little
harder to describe in writing. It is the earliest Early start of the
task in question's successor tasks minus the Early Finish of the task
in question. (Pretty confusing, I know).
They are used for somewhat different purposes. Total slack tells you
how much you can either put off a task or extend it's duration without
impacting the project end date. In the real world, it is often traded
to remove Resource Overallocations or to relieve pressures elsewhere
in the schedule.
Free slack is considered usually with regard to resource scheduling.
If one task slips and forces another task(s) to slip also, even though
it doesn't effect the project end date, it forces you to reschedule
the resources on the downstream task(s). Sometimes this is easy,
sometimes nearly impossible.
These numbers help you to properly plan and execute the project and
also tell you, to some degree, how tightly scheduled your project is,
and, by inference, relative schedule risk.
Hope this helps a bit.
David G. Bellamy
Bellamy Consulting
Free Slack is the amount of time a task can slip and not effect any
other task.
Total Slack is the amount of time a task can slip and not effect the
end date.
Total Slack is calculated as described above. Free slack is a little
harder to describe in writing. It is the earliest Early start of the
task in question's successor tasks minus the Early Finish of the task
in question. (Pretty confusing, I know).
They are used for somewhat different purposes. Total slack tells you
how much you can either put off a task or extend it's duration without
impacting the project end date. In the real world, it is often traded
to remove Resource Overallocations or to relieve pressures elsewhere
in the schedule.
Free slack is considered usually with regard to resource scheduling.
If one task slips and forces another task(s) to slip also, even though
it doesn't effect the project end date, it forces you to reschedule
the resources on the downstream task(s). Sometimes this is easy,
sometimes nearly impossible.
These numbers help you to properly plan and execute the project and
also tell you, to some degree, how tightly scheduled your project is,
and, by inference, relative schedule risk.
Hope this helps a bit.
David G. Bellamy
Bellamy Consulting
M
Mike Glen
Hi Jamie,
Please see FAQ Item: 42. Guide to Network Analysis
FAQs, companion products and other useful Project information can be seen at
this web address: http://www.mvps.org/project/
Hope this helps - please let us know how you get on
Mike Glen
Project MVP
Please see FAQ Item: 42. Guide to Network Analysis
FAQs, companion products and other useful Project information can be seen at
this web address: http://www.mvps.org/project/
Hope this helps - please let us know how you get on
Mike Glen
Project MVP