I have a project task that has a variable length, for example +/-.

[danc]

I have project tasks that do not have an exact duratation, but rather a
proposed window. How to I enter that into MS project?

 

[davegb]

Actually, all durations, until they've occured, are estimates. In
general, when you put in an estimate, it's just your best guess,
preferably, in my experience, on the pessimistic side.
Another option, though now know to be statistically unsound, is called
PERT Analysis. In this methodology, you make 3 estimates of each tasks
duration, optimitic, pessimistic, and most likely. You plug them into a
standard formula, which takes a weighted average, giving the most
likely estimate 4 times the value of the other 2, and used that average
as the duration estimate. In Project, you can implement this by turning
on the Analysis toolbar, then clicking on the PERT Analysis button,
which brings up the PERT Analysis toolbar. From there, you can enter
the 3 estimates either in a form or a table. When you click on the
"Calculate PERT" button, and it will do the math and enter the result
into the duration field.
I can't stress too strongly, that this method is terribly outdated and
misrepresentative of the real implications of multiple potential
duration estimates. The statistically correct way to do this is to use
Monte Carlo analysis, and there is software out there that does it. It
gives far more meaningful results than PERT Analysis. I'm not enought
of a statistician to explain it all, but have read enough to be
convinced that it is so.
Best of luck, however you proceed.

 

[Steve House [MVP]]

Tasks by definition have an exact duration. You might not know precisely
what it will be ahead of time but it is exactly however long it takes the
resource doing the work to complete the task's deliverable once he starts on
it, no more and no less. One of the reasons for using scheduling software
such as MS Project is help you get a handle on such things as duration
estimates, by recalculating task lengths based on such things as resource
availability and other responsibilities they may have at the same time as
they are doing the work on their assigned tasks. I can't emphasize too
strongly - the task duration is *never* the "window of opportunity" during
which you need to get the task done, instead it is your best guess as to the
concrete amount of time the resource will physically be working on the task
between when he starts work on it and when it's finished. So in your plan
you should use an estimate, based on the history of similar tasks from prior
projects, your expertise in the subject matter of the project, perhaps
conversation with the resource who will do the work, etc, as the expected
duration for scheduling purposes. You might revise it later on as the plan
matures.