Overallocation essentially means that the resource is double booked, that
is, scheduled to be in two places at once. The resource allocation is a
measure of the rate at which work is accomplished, NOT a measure of the time
the resource was allowed to do the work. Consider the case of a single
individual who works an 8 hour workday. 100% means that he is expected to
deliver 8 man-hours worth of work on a task over the course of his 8 hour
workday. Of course, the duration of the work is irrelevant - 1 man-hour
work in 1 hour of time, 6 man-hours work in 6 hours of time, or 16 man-hours
of work in 16 hours of time all represent 100%. Working at full speed Joe
can produce 20 widgets an hour. Regardless of how long he works, if he's
produced 20 widgets for each hour he worked he's worked at 100%. But if
he's only made 10 widgets per hour, then he's been working at 50%. The
reason overallocation is an issue is that there is no way at all, a physical
impossibility, for him to ever make *more* than 20 widgets an hour. So if
your project schedule is based on the assumption that he will work 8 hours
and produce 500 widgets you've got a problem - he's booked to produce >300%
of the most work he can possibly do - and that's what an overallocation is
all about. Overallocation occurs whenever the number of man-hours (or
man-minutes or man-days) of output you expect the resource to produce is
greater than the time you expect him to do it in. If I work an 8 hour
workday it is physcially impossible for me to accomplish 16 man-hours of
work during that day. On the other hand, I might produce less and it is
totally possible for me to only get 4 man-hours of work done over an 8 hour
time period.
When you enter a resource you specify his maximum availability and that is
what determines what Project considers overallocated. For convenience we
think of it in terms of the percentage of hours out of his calendar workday
that he is allowed to work on project tasks but what it really means is how
much of his total possible energy over any arbitrary time period you can use
in your project. If someone has a maximum of 75%, that means he can do 6
man-hours of work for us over an 8 hour duration. If we book him 100%,
we're expecting him to do more than he possibly can do in the time we've
allowed him to do it and we must either reduce the amount of output we
expect, give him more time, or add someone else to pick up part of the load.
