The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 46

INTRODUCTION
Suppose that you meet two friends and decide to go on a hunting trip. One friend
will get the food, the other will get the hunting gear, and you will prepare your Jeep
for the trip. After each person finishes his or her assignment, you will drive together
to the hunting location. Also suppose that the first activity (getting the food) requires
2 hours, the second activity (getting the hunting gear) requires 3 hours, and the third
activity (getting the Jeep ready) requires 4 hours. If all three activities start at 8:00 A.M.
(when each of you leave to perform his or her task), the following activity (driving to
the hunting location) cannot start until all three of you return (i.e., when all three
activities are completed). If everything works according to plan, the first person will
return with the food at 10:00 A.M. ; the second, with the hunting gear, at 11:00 A.M. ;
and you, with the Jeep ready, at 12:00 P.M. (see Figure 4.1).
Now, ask yourself the following question: “Which activity 'drives' or controls, the
schedule?” The answer is the third activity (preparing the Jeep). Any delay in your
return will delay the trip past 12:00 P.M. In contrast, your first friend will have 2 extra
hours to “play with,” and your second friend will have 1 extra hour. They can choose
to use the extra time however they like. For instance, they can do the following:
• Get an early start, finish early, and then take a break
• Get a late start (10:00 A.M. for the first person and 9:00 A.M. for the second
person) and finish at 12:00 P.M.
• Take breaks between work periods
We call this extra time float. Your friends will have to watch their time carefully
so that they do not “waste” more time than the float they have: 2 hours for the first
person and 1 hour for the second person. Any delay past these float times will result
in a delay in the entire schedule. You, however, have no float. You are running on a
tight schedule. Your activity is critical . This scenario provides a simplified example of
the critical path method (CPM).
Figure 4.1 Bar chart for introductory hunting trip example