The first step in determining the basic schedule is
to do a forward pass. The forward pass gives you the
total duration of your project. You start with the very
first activity and plug in a zero for its early start date.
Then add the duration to the early start date to get the
early finish date. The early finish date for an activity
becomes the early start date for the next activity.
Notice that activity 1050 in figure 2-13 had two
preceding activities (1020 and 1040) and you chose
the larger of the early finish dates (11 vice 10).
Remember to add any lag between the activities
(between activity 1070 and the next activity there are
3 days lag time). Lag times are mandatory wait
times between activities. A common example is
concrete cure times. Cure times require you to wait
several days to several weeks after placing concrete
before you strip the forms. Follow the following two
equations through the network in figure 2-13:
Early Start + Duration = Early Finish
Early Finish + Lag (if any) = Early Start (next activity) *
Look at the network in figure 2-13, the early start
and finish dates for an activity depend on the number
and duration of the activities that have to be done
The next step in determining the basic schedule is
a backward pass. The backward pass determines your
critical path. You start by taking the early finish date
for the last activity and making it the late finish for the
last activity. For each activity, subtracting the duration
from the late finish date will give you the late start
date. The late start date will become the late finish date
for the preceding activity. Notice that activity 1040 in
figure 2-13 has two follow-on activities and you took
the smaller of the late starts (11 vice 12). Follow the
equations shown through the network in figure 2-13.
For any activity where the early start is the same as the
late start and the early finish is the same as the late
finish, that activity is critical!
Late Finish Duration = Late Start
Late Start Lag (if any) = Late Finish (preceding
Total float is the number of days an activity can
be delayed without delaying the project completion
date. Looking at activity 1020 in figure 2-13 you see
that it could finish as early as day 10 or as late as day
12. The 2 days of leeway between day 10 and day 12
in activity 1020 are called total float. To calculate total
float you subtract the early finish date from the late
Figure 2-13.Typical network.