The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 74

Activity Duration ES EF LS LF TF FF

A 2 020200

B 5 494900

C 6 287 350

D 6 9 15 13 19 4 0

E 7 12 19 12 19 0 0

F 4 9 13 13 17 4 4

G 5 8 13 15 20 7 6

H 0 9 9 9 9 0 0

I 8 19 27 20 28 1 1

J 7 15 22 22 29 7 7

K 1 29 30 29 30 0 0

Lags and Leads in Computer Software

In software scheduling programs, you may assign different calendars to different activ-
ities. For example, activity A follows a 5-day calendar, but activity B follows a 7-day
calendar. Which calendar does the lag follow? Theoretically, it could follow A's calen-
dar, B's calendar, or its own. Some software packages do not give the user the choice,
and some do. You need to know which calendar the lag follows and whether you have
any choices.
One interesting scenario (Figure 4.12) may lead to a pitfall: there is a 2-day lag,
but the scheduler has timed activity A to finish on a Friday without inserting a lag. The
next activity, B, will start on the next business day (i.e., Monday). This leaves a “natural
lag” of 2 days: Saturday and Sunday. A good example of this is concrete placement.
Let us assume activity A to be FRP (Formwork-Rebar-Concrete) Columns. Activity B
is Strip Forms, Columns. The schedule works well unless a delay forces activity A to
finish 1 or 2 days later. Activity B will then start the next day without any lag. Ideally,
a 2-day lag must be entered. If there is a choice for the lag calendar, we should choose
the 7-day calendar. If not, we can create a 2-day activity (Wait for Concrete to Set)
in lieu of the lag and assign the 7-day calendar to it. Such an activity will have no
resources or cost assignment.

Figure 4.12 Two-day lag