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Products > PSOP > Documentation
PSOP Overview
A project consists of a collection of activities, relationships between
those
activities,
constraints on the activities, and the resources assigned to work on
the activities. Resources may have limited availability.
For example, if there is only one welder on staff, we would
set the availability of welders to 1. If we have two activities
that require the welder, a schedule where both activities occur at the
same time would
be
over-allocated (we sometimes also say the welder resource is overloaded).
Consider the following simple example. A father and son set out
to mow the grass and tidy up the path in their yard. The son is
too young to use power tools so the father must do the mowing and
edging. The son can tidy the path, but only after the father has
done the edging around the path. Such a project might be
represented in Primavera as follows:
Edging the path is expected to take 2 hours, tidying the path should
take 3 hours, and cutting the lawn should take 6 hours. Since the
edging must happen before the path can be tidied there is a Finish to
Start relationship between the edging and tidying tasks. We have
specified a project deadline of 5:00 pm.
The schedule above is what Primavera produces when schedule is clicked.
This schedule requires the the lawn to be cut at the same time
the path is edged. Since both tasks use the same resource (the
father) and the availability of that resource is 1, this schedule is not
workable and must be leveled.
If we level the project in Primavera with the default leveling
options, it produces the following schedule:
While our resources are no longer over-allocated, this is clearly
not the optimal schedule; in fact, it finishes at 7:00 pm and misses our deadline
of 5:00 pm. It would be better to schedule the Edge Path activity first, so that the son can tidy the path while his father is cutting the lawn.
So what went wrong? Primavera levels
by sorting all tasks according to their leveling priority. By
default the leveling priority is "Activity Leveling Priority", a user
specified priority for each task. Since we didn't specify a
priority for our tasks, Primavera simply uses the activity IDs as the
priority order. Using this order Cut Lawn
comes before Edge Path and Tidy Path because the tasks are simply
sorted alphabetically by their ID. Thus Cut Lawn is scheduled first and placed at 8:00 am (as early as possible). The next activity scheduled is Edge Path, but it can't happen at the same
time as Cut Lawn because it requires the same resource (the father). Therefore, Edge Path is delayed until 2:00 pm which in turn delays the Tidy Path activity.
What we need is a different priority order. Primavera offers
several possible priority orders:
Trying some of the more obvious sort orders, "Total Float", "Late
Start", or "Early Start" doesn't work on this project either.
Choosing "Early Finish" will produce the optimal schedule:
This optimal schedule finishes by 4:00 pm, 3 hours before the
initial schedule Primavera produced.
Manually trying different priority orders worked on this tiny project,
but isn't a good approach in general. On this three task
project there are only 6 different priority orders so we can keep
trying until we find one that works. However, given a relatively
small 10 task project there are 3,628,800 priority orders! For a
more realistic 100 task project there are over 9 x 10157
different priority orders (that's a 9 with 157 zeros after it)!
Not only is it not possible to try all the different priority
orders manually, it's not even possible with a computer. If you
had a computer that could try one billion different priority orders per
second, it would take about 3 x 10141
years (for comparison that's much, much more than the age
of the universe which is only 2 x 1010 years).
What is needed a program that intelligently determines the
priority orders that are most likely to improve the schedule and a
computer that can try many such orders. PSOP uses On Time
System's patented technology to identify the best priority orders.
The result is schedules that are shorter and have less
over-allocation, saving both time and money.
Additionally, PSOP offers many other
advantages.
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